I go with GT2 actually, now, the controls, sound and graphics are heads and tails better in GT3 (which goes without saying), but GT2 brought alot more to the table. More cars, more tracks, more modifications available (namely, the Racing Mods). It was also the first time that Rally was introduced, and they had planned on adding in Drag Racing. Now, I myself think that the Rally and/or the Drag racing is fairly useless (Well, I can't say that for sure 'bout the Drag Racing, since it never REALLY has existed). It's just not fleshed out enough to make it a viable option. That being said, it doesn't take away from the Developers ideas - innovation.
GT3 (while incredible) is just more of the same. A wise man once said "If you can't innovate, duplicate" (Ok, I always say that, whenever I'm rippin' off some code on the 'net, and I think I'm the only one, but I'm hoping it catches on)
What's the only new thing in GT3... Monaco, Tokyo? (I don't recall if Tokyo was in GT2 - it's been a long time...)