Honestly it depends on how you play either of them.
if you go into those games with just a mindset of beating it, both are going to be short.
Collection wise, 3Dland is the way to go. If you take your time, you can put some hours into the game.
however, I honestly like NSMB2 more. The game can be stretched enormously if yoj focus your time into finding star coins without a guide, the level design is a bit simple but entertaining, and I would STRONGLY suggest it if you get alot of streetpasses and someone has the same game. There is a great sense of satisfaction in destroying peoples records in coin rush.
and that would be my other point of this argument, is the DLC level packs. Theyre only for coin rush though, not the main game. However, if your the kind of gamer that likes to top high scores, you can get a lot of satisfaction from toppingg coin totals in coin rush. And some of the DLC packs are very challenging.
Last note, both of the 3D effects are really good. 3D land is alot better then NSMB2 in terms of 3D of course, but NSMB2 does a decent job as well.