I don't like Sonic the Hedgehog very much. The marketing goes on and on about how fast Sonic is, but I call it "Sonic the Hedgehog: Waiting for platforms". You run real fast, stop, wait for some moving platform, or for a slow spiky enemy to move out of the way, or for the fireball to go back down, etc. It's all stop and go, and if I go too fast I wind up running into a pit, or spikes or something. It takes me nearly 20 minutes to finish the Marble Hill Zone alone. I'm sure if you memorized the game, you can blast through it in five minutes, but as it is I just don't have the patience for Sonic. Mario may be slower, but the pace flows a little smoother. It could also be that I just suck at Sonic the Hedgehog.
I enjoyed Super Mario World back in it's day, but It's pretty monotonous to me now.
So, between the two.... I'd keep my NES.