Gymkhana is about where I stopped playing Dirt 3 and started playing Dirt 2...
As I have progressed further in Dirt 2 I definitely feel like the graphics do a better job of setting the atmosphere for the race and the general feeling. Dirt 3 just has a very clean look and I'm not sure I like it.
I also feel that the events in Dirt 2 are more varied and there is more choice as to what you want to race and when you actually want to race it. The tracks in Dirt 2 are better too, can't put my finger on it but I think they're just a little better designed and there is more "feel" to them, more stuff going on. Of course the interface in Dirt 2 is a bit of pain but I am having trouble putting it down. I definitely feel more of a gritty (and fun) rally feeling from Dirt 2, I haven't had this much fun playing a racing game since NFS Underground.
I hope they're making Dirt 4 with "next gen" graphics and the best bits from both 2 & 3, would be awesome.