From my games against MdrnDayMercutio (IDK if he posts on Sonic boards), it seems that the fact that Sonic can't reliably approach MK from the side (Fair stops all approaches, or at least that's what it looks like, or from above (Uair stops all approaches it seems), or from below (MK's Dair) and MK's CPs are much stronger. Plus, on PS1, MK can gain a % advantage and plank under the ledges while Uairing and I don't think Sonic can do more than try to Bair off the stage. I think the 80/20 the MK board has is high, but from the one Sonic I played against, I'd say it's like 75/25 (assuming Corneria and GG are banned in the competition, which I support). My experience is far from diverse, however. Are you aware of moves that can get through MK's aerials? Or do you have to rely on punishing.
I can bring my Wii if you have a TV... I prefer my Wii anyway since I painstakingly picked every song that is the best to Brawl to on every legal stage.