I love both of those games. I think I prefer 3D Land because it's nostalgic and has the right amount of challenge. Obviously it's easy through most of the game, but the special worlds are better. Then of course there's the final challenge, which I love.
A bit hard to compare two games that have 5 years between them and when one of them is a platformer and the other is a 3D game, but I think I say Super Mario 3D Land..
New Super Mario Bros.
The Worlds, Levels, Power-ups, and Especially bosses are better.
It's also funner to pick up and play, and the levels seem more replayable than 3D Land's
But probably I am leaning more toward Super Mario 3D Land, because of its 3D factor. It's new, and it feels fresh. With NSMB I am getting the feeling (more so than with 3D Land), that I have seen and done everything here before.