Would Sonic be different if he was created by Nintendo?

I don't know. I think he would though. I don't think he would have much personality seeing as how Nintendo leaves most of us in the dark about character personalities (examples: Mario, Link, Kirby, etc.).

He probably wouldn't talk that much, none of the other nintendo people do. He would probably run around and say "Hedgehog!" and crawl on all fours, just like Yoshi.
(I wonder if he would have a shell on his back too... Hehe)