There's a thread like this in the Fox forums just like this, the name is just reversed. Anyways, My thoughts:
Fox and Sonic are both good characters, but when it boils down to it, Fox has the upperhand, but not by much.
Yes, Sonic has got speed over Fox. But Fox's attacks strike faster, and overall I think Fox's attacks have more priority than Sonic's. Fox also falls way faster. I think he still has one of the fastest falling speeds in the game.
Fox can combo like crazy too and it racks up some serious damage. It's delicious. For example, Fox's uptilt lock can rack up 40 damage at least on any character, and that's just one of the many lock-on's/combos Fox has to offer. But at the same time, comboing and approaching can be risky with Fox. As for Sonic, I don't believe he has much combo potential. I've seen several F-airs pulled off one after another, but that's all I've seen. Not much combo potential going for Sonic.
As for recovery, you guys are absolutely right, Sonic does have the better recovery. Regardless though, if you knock Fox off the stage, chances are he's coming back. He's got Fox Illusion, Fire Fox, and he can even shine stall if need be. All can get him to the stage. So...it's not really a big deal.
I'll hand it to Sonic on approach though. Fox's approach are garbage, and Fox players know this, and then that's where blaster comes in handy. Fox's blaster might to appear to be lousy, because it doens't stun your opponent (Like Wolf's and Falco's does), but I believe to Fox players it's nothing more than a tool to make you approach Fox, so us Fox players don't have to risk approaching ourselves. The fact that it gives damage is a great bonus.
Fox can kill early, Sonic can't. Fox's upsmash *****, anywhere past 95 on Sonic, then consider him screwed in the butt, because the upsmash will send him to oblivion. Let alone that if you have atleast 110 damage and if Fox dairs you it'll make you trip and he can just Upsmash you there.
Fox is a lightweight. Lol dunno why. Pikachu is heavier than him FFS. So Sonic does have the weight advantage.
Lastly, I don't really know about Sonic, but a smart Fox that can play excellent mind games with there opponent will be very succesful. Fox has great agility and quickness, putting these together to your advantage definitely has it's benefits. If Fox is predictable, he seems to suffer a lot more than other characters do from it. Whereas Sonic I don't think you have to worry about it that much.
This is all I can think of.
My conclusion: Fox > Sonic, but not by much. Don't get me wrong, Sonic has a lot of potential, but his is only slighty less than Fox's, due to Fox's insane combo'ing, priority, power (aka upsmash) and if used right, quickness.