hmm..SNES/ Genesis era Id say Sonic, N64 and beyond though Id say Mario. Colors and Generations are fun, but overall Mario just transitioned alot better into 3d imo. I still play them both so it doesnt really matter I guess lol.
I grew up with the old Sonic games and preferred them to the Mario games I played as a child (Super Mario Land 1&2 on the gameboy), but I have to admit that his modern games have all been awful. That said, Sonic Colours and Generations seem to be getting him back on the right track, so I'm really hopeful they'll keep up the trend and restore him to his former glory.
And yeah, I admit that the Mario Land games might not be considered "official" Mario games.
But if you had asked me that 5 or 6 years ago I would have said Sonic as I used to be a massive SEGA fanboy but not any more. His last 2 shit as hell games killed my love for Sonic forever. I never want to play that crap again.
Mario is just rehashed to death, re-released to death kiddie crap of the first order. I wouldn't touch any Nintendon't shit with a barge pole. The last time I played a Mario game was almost 25 years ago.