3D is much faster than 2D, so it gives much more power graphically. It's harder to learn but has a ton of potential. Of course, you could always make a 2D game using the 3D pipeline by making all of the objects flat 2-polygon planes and setting the field of view to 0.

Gameplay-wise, they are both good, but 2D is pretty limited in design. It would be kind of lame playing a game like Half-Life (2) in a 2D world. It just isn't cut up for the job.

Yeah, that worked very well for some games. Remember Kirby 64? The camera would rotate on some parts but for the most part, the gameplay remained 2D.