Hmm. That's a toughie. It's especially tough because in my experience, good creativity and good mechanics are NOT mutually exclusive. Far from it. Language is the way we present our stories, and the presentation is important for getting the story right.
To tell you the truth, I seldom see a "great story with terrible mechanics." (I have seen a few perfect manuscripts with boring stories, a few fairly good stories with terrible mechanics... mostly I've see okay stories with okay mechanics, alas.)
First, I guess I'd like to say-- no editor is looking for a perfect manuscript. Editors assume that there will be a few typos, a few infelicitous phrasings, some small format problems. No editor starts hyperventilating at the prospect of working with a writer who is a little less than perfect on every page. Got to justify existence, don't we? Perfect writers don't need editors!
So never worry that the editor is going to read 200 pages and on page 201 discover that misspelling and decide to reject. That won't happen. But what if there are four misspellings on the first page? What should the editor do then? (When encountering a lot of mechanical problems right away, I started sending back the submission without reading further, and saying, politely I hope, "I know you would like another chance to edit so that this is easier for me to consider." Generally, the writers have all thanked me for the chance, though who knows what they're really thinking. :)
One thing I do have to point out is that there really isn't an either/or here. Creativity might be messy at the creation stage, but I know very well there is no need for a good story to be messy at the submission stage. Most good stories go along with at least adequate mechanics, because the writer cares enough about presentation and narration to work hard at things like sentences and paragraphing. Most good writers don't assume that "story" is just "idea," but understand that ideas are developed in scenes which are made up of causally linked passages which are made up of paragraphs and sentences.
A mechanically inept manuscript is, in my experience, more correlated to inept development of the central idea or plot. I might see, in a mess of a manuscript, a good plot idea, or a glimmer of brilliant characterization. But that's usually all there is-- an idea, a glimmer. The execution and development aren't done well, particularly at the scene level. Why, well, interestingly, I think, there is "story grammar" and "scene syntax." Just as in a sentence or paragraph, stories and scenes have relationships that are shown in the structure or design. If the writer doesn't get that this pair of sentences shows a causal relationship:
He lurched forward, his mouth open.
I got out of the way.
Then probably the same writer isn't going to design a scene developing the cause/effect relationship between bigger events, although those events might be terrific.
Have I ever seen a great story with lousy mechanics? Yes, but mostly with my college students. If they come from a storytelling family or culture, often they get the story grammar talent with mother's milk. They've been surrounded by great stories all their lives. But usually this is oral storytelling, and often their ability to write it down is limited. We see this a whole lot with non-native speakers, especially those who left their home culture before high school, so that they aren't "writingly fluent" in their native language either.
We also see this in native speakers who didn't have adequate educational experiences (or who weren't paying attention... the class clown comes to mind-- usually he's a great storyteller.) The issues are usually spelling and punctuation, not sentences-- that is, it's really the -writing- stuff, the letters and punctuation marks which aren't clear in spoken English that cause the problem. Sometimes word choice is lacking too, especially in non-native speakers-- they just don't yet have the vocabulary. But they do have the ability to describe setting and people, to design scenes for maximum drama, to select the telling detail.
I had two students like this in one semester. They both would have gotten an A if I taught speech. As it was, one got an A, the woman who wrote very affectingly about her grandmother being diagnosed with Alzheimers the same week the writer found out she was pregnant, and how that baby ended up helping the grandmother keep her speech long into the illness. She worked closely with me and a tutor to find the mechanical problems that got in the way of the story presentation.
The other was a young man who wrote (this was a kind of emotionally wrenching semester) about getting to the hospital just a few moments after his mother died, so he couldn't say goodbye. The urgency of the journey across town -- wow. Beautifully structured with great suspense. But he didn't have the time to transform this great story into a great paper, and didn't get as good a grade (though I made sure he knew that he had all the right stuff and just needed to go this additional step, and I hope he did in the future).
So I know it's possible to have great story/bad mechanics-- but I have to point out that these were students in freshman composition, each coming out of an oral tradition that rewarded great story design and impressive vocal performance (which they had-- as I said, they both would have gotten As in a speech class). Transferring that to written language is a separate process.
But if you're submitting a written manuscript to a book publisher, well, it's expected that you are as adept at the tools of the craft. Those students might not know so much about punctuation and other elements of written language, but they did know how to use vocal expression and pauses and body language as they told their story. (They weren't so great at first at transferring that to writing, but they really did have the vocal tools for storytelling.) If you choose to write this story, it's kind of expected that you would use the written-language tools adequately.
Now, as I said, minor errors are not the issue here, and I think writers who get upset when I say I want a mechanically adept manuscript might think I mean no typos. But what I mean is-- well, truth is, most of you would be shocked to see a story with dialogue like this:
He said "Joni Im sorry about your cat's.
She said don't worry about it. There probably hiding in the garage"
That's not actually the sort of "messiness" that goes with creativity and great ideas. But that is what we see a lot. And that sort of leaden presentation means the voice is usually leaden too. Voice isn't just about word choice-- it's developed through sentencing and punctuating too.
So... let's say you are like my students, trained by tradition and upbringing and talent to be a great storyteller but not a great practitioner of the written discourse? You know what I'd suggest you do? I'd suggest you dictate your story into your phone recorder, and hire a good secretary or transcriber to type it. (If anyone knows of a speech-to-text app which does a good job, let me know. My "twalking" -- talk-walking- recordings always end up as gibberish as text.)
Many transcribers have been, uh, quietly editing their clients' prose for years, and know how to turn your dictation into an fairly adequate manuscript. (When I worked at the late lamented Grammar Hotline, most of our callers were secretaries who were interested in getting the grammar right, or in proving to their boss they were right, and they usually were.:) If the problem is getting it from oral language to written language, it's probably easier and cheaper to hire someone to make the transfer yourself.
Now that I think of it, the corollary -- the perfect manuscript and boring story-- happens more often. That's because you can hire someone to turn oral language into written language -- same words, after all, and it will still be YOUR story, not the transcriber's. But if you hire someone to design your scenes, deepen the characterization, create a suspenseful tone, structure the events-- it's not really your story, is it? All those things ARE story. (And that is why people hire ghostwriters, I guess.)
Thinking back on perfectpunctators/lousystorytellers... I have seen that too. I used to write Regency novels, a subgenre that attracted a lot of English teachers and librarians (it's set in the time of Austen, see). And when I'd judge a Regency contest, I'd frequently get an entry that was well-written on the basic word level, but lacking in story grammar. They knew how to write a sentence, but couldn't flesh out a character. They knew how to punctuate dialogue, but not how to make it sound authentic. The story would never be insane (that's much more likely with the messy manuscript, and yeah, I've seen that a lot too), but it would be "by the numbers," often using conventional situations (ballroom scenes, mistaken identity) with nothing fresh added.
In a contest, this would often score sort of on the high end of mediocre, but never win. And really, I don't have a quick solution ("hire someone to type it") here. The problem is more global, more personal-- that is, the writer probably doesn't have a great imagination and/or an innate or learned sense of story grammar, and you just can't hire that. (But I do think these would be great transcribers for the oral storytellers out there! :)
So... which of the two (messy but good story, clean but boring story) would be more likely to be published? Hmm. Well, of course, when we pick up a published book, we're seeing an edited version, not the original submission. So there might be plenty of previously-messy books that have been wrestled into rightness by a pair of editors and a proofreader, and we'll never know unless we get the editor drunk. ("You know that writer of mine who made the NYTimes list last week. Boy, you should have seen the manuscript when it came to me. One long sentence, the whole first chapter. I kid you not. You're buying the next round, right?") Notice that this requires a lot of time and energy from the editors and money commitment from the publisher, so a damn good story is required, not just a good story, to elicit that much effort.
But we certainly all read boring but well-written books. They're well-written enough that we don't take them back to the bookstore and demand our money back, or post nasty reviews on Amazon. We don't feel passionate enough about them for that level of response. Meh... we sort of wonder why this book was chosen out of the many the editor must have read that month. (Probably the original book for that slot didn't come in on time, so they needed a book to fill the gap, a book that didn't require much work to make presentable, and this one landed very cleanly on the desk at just the right moment. See why it's a good idea always to send in a clean manuscript? "Doesn't need much editing" is maybe not the fulsome compliment you were hoping for, but there are times when that's exactly what the publisher wants in a book.)
Well, anyway, we should all strive for great story/great mechanics. Figure out our weakness and what to work on to overcome it, but maintain our strengths too.
I'm remembering a query I got from one of those meticulous types, the one that made me really WANT to buy out of pity-- "I always make deadlines. I always deliver a clean manuscript. I have worked as a proofreader for a decade"-- it was sort of sad. Imagine an epitaph: "She always made her deadlines, including this one."