SMB3 seems better (in terms of level design and power-ups), but SMW gives me better feelings. Actually, one advantage SMW has is "secrets", which is quite a big deal. And Yoshi is a big deal, too, even if he kind of breaks the game. SMW, it is.
"Ah, a party! We haven't had one of those. It could be fun! So... what is a party?"
"You drink punch and eat CAKE! ...I think."