Since I've yet to swipe my fingers on a tablet or phone that takes advantage of 3D technology, I can't conclusively form an opinion on 3D mobile devices. Gimmicky or useful? Whatever your opinion, the phones will soon be in a store near you.
As far as which phone can better reproduce 3D images, Slashgrea favores HTC. "Yes the high-res display turns a little grainy when 3D is activated but the 3D effect seems better on the EVO and the grainy effect is also less and that helps with the possible headache 'issue'". Take a look at the hardware, as well as 3D capabilities, of both units in the video below: