Working with an editor before submitting a query
Let me preface this by saying that my experience has only been with the editors my publishers have provided.
Let me also ask if you're ready for an editor. The reason I mention it is that an editor's job isn't to take unprofessional writing and make it publication ready. If you aren't fully up to speed on structural issues (such things as knowing why a scene on the page is a very different thing from one in film) and the language of the profession, you'll not understood what's meant when the editor says that a certain section of prose is too passive, or that there's a break in POV, etc. Editors correct grammar and simple items (my latest editor, for example, hates leading tags, and eliminated many of them. We're still doing battle on that point). They expect you to fix most problem areas after they point them out.
The reason is simple. If they were capable of telling you how to make a given story publication ready with any assurance of a sale, it would be faster and more profitable for them to write and sell their own work.
The job of an editor is to be an educated eye and notice all the little screw-ups you miss because you're too close to the work.
As for a "developmental editor," in my experience these are usually retired editors with experience in the publishing industry, who are lending their expertise in "getting over the hump," so to speak. It would be a hugely expensive way to learn how to write, especially when a book many feel is the best one available on writing fiction, Dwight Swain's, Techniques of the Selling Writer, is available for less than the cost of a Saturday night out. And Jack Bickham's Scene and Structure, a book nearly as good, can be found free in many library systems.
I worked as an editor at a small publishing company for two years, and let me tell you, getting a manuscript that wasn't ready wasn't fun. An editor should be a guiding hand. Someone who points out mistakes in grammar and punctuation, and makes a few comments on story and characters. Whenever I got a manuscript in my hands where my notes about what you're looking for are more than half the word count of the manuscript? That's not what I should get as an editor. A third or fourth draft at least is what you should send an editor.
I say go to the query first. Maybe find a critiquing group. They can give you a lot more detailed insights to your writing, and you can get a lot more voices telling you what they think. There are a few good ones if you search. Maybe get some advice, then put your story on here. But an editor should be last.