While Stan is right--that Spider-Man in any form would not exist without him--Ditko is also right that THIS Spider-Man would not exist without him. And that's what Stan doesn't seem to want to acknowledge--that Spider-Man as he exists today could not exist without Ditko's contribution.In the "In Search of Steve Ditko" documentary, Stan basically says the same thing -- that's he's happy to call Ditko the co-creator if it means so much to Ditko, but he still believe he himself is the creator. I haven't seen the Kevin Smith interview, but in "In Search of Steve Ditko" he doesn't seem ungracious or like he's being a jerk about it, he just honestly believes that he himself is the creator. When pressed about the fact that had he given the assignment to a different artist with different ideas, it would have been a completely different character that might possibly have failed, his answer is that in that case, he wold have created a character that was a failure.
I can't say I entirely agree with Stan's point of view, but it's understandable at least because he's not really claiming credit as the writer but as the editor in chief of Marvel. From Stan's perspective, of course, his job is to put out the best comics and that includes deciding which artists best fit which ideas or characters. The fact that he assigned the Spider-man strip to Steve Ditko, who developed it into what Spider-man became, to him means that he is the creator because as editor he had final say over all the creative decisions that went into the strip.
Interestingly, the genesis of Spider-man as I understand it both undermines and supports this viewpoint. What I've read from several sources is that Stan originally assigned Spider-man to Jack Kirby. Kirby turned in a much more straightforward superhero who, to Stan's eye (and perhaps others in the company) bore too close a resemblance to one of Kirby's old characters, The Fly. Whether to avoid litigation or just for creative reasons, Stan decided to scrap Jack's version and reassigned Spider-man to Ditko, who came up with the version we all know and love.
It's obvious from this that Ditko is clearly the co-creator at the very least, if not the outright creator, of Spider-man. On the other hand, Lee's point is also valid; he could very well have gone with the original Kirby concept for Spider-man instead, but as editor decided it needed tweaks to be successful.
It's really tricky to try and assign credit in a system where not only are roles not defined to being with, but where multiple overlapping roles are held by the same person -- where does Stan the editor end and Stan the writer begin? Stan's position is even more understandable considering how many years prior to the Marvel Age that he spent as the company's main or only writer, hiring and using whichever artist best fit his vision for each story. Even though he was giving them more artistic leeway once the Marvel Method came into being, I'm sure that from his viewpoint it was just an extension of the Atlas days, where the artists were there to bring his vision to life.
Anyway, I'm firmly on board with them both being co-creators and I think Ditko has a legitimate beef, but I can understand why Stan has the position he has, and I don't think it's a matter of being petty or ungracious or stealing people's credit but rather that he honestly believes himself to be the creator of those characters and the artists to be work-for-hire freelancers that he hired to draw his concepts.
My two cents.