I was disappointed with Mario 3D Land until I got to the special levels and I was hooked. It needed the crack up in difficulty to stop it feeling like running through a unstimulating obstacle course. Ultimately a great game which I loved and has my favorite browser boss fights of any Mario game.
3d World built on 3d Land, had a great multiplayer, and got harder a little faster. But the novelty was less fresh as I'd played 3d land before it so I never liked it quite as much. I thought some of the post game levels in 3d World were some of the toughest Mario levels in a long time. I probably played many more hours of 3d World because of the multiplayer. I suspect it would be my prefered game of the two if I'd played it first.