I look at it like this, Super Mario Kart has the better single-player, and Mario Kart 64 has the better multiplayer.
But I do have one big problem with Super Mario Kart. . . no Wario.
As for Mario Kart Advance, I want more everything! More levels, characters, weapons, modes, etc., etc. I've always loved the Mario Kart games, and I'm hoping Mario Kart Advance is the best yet. It'd be great if Nintendo took SMK's best aspects and MK64's best aspects and put them into one game. . .
As long as Waluigi isn't in it, I'll probably like it :)