Taking only the games themselves, 3D World is the better game with it's fleshed out world and more elaborate levels and bonusses.
However, 3D Land is better at being a handheld game than 3D World is at being a console game, if you catch my drift. 3D Land is something you would expect as the 'main feature Mario platformer' on a handheld, leaving a positive taste, while 3D World is less of what you expect from the 'great 3D Mario adventure' on a console, given there could also be an epic 64/Galaxy-like game. This makes it feel like this would actually be a side-game, leaving a slight negative taste.
I'll vote 3D World however, because black/white, it's the better game.