I'm a huge fan of Crash mode, so Legends gets my nod. Not to say that Dominator is bad, but the lack of crash mode really sucks. Plus, to me, it seems more finicky when it comes to crashes in races and such. Both have a good soundtrack, although I prefer Legends (but Dominator's soundtrack definitely fits the atmosphere of the game...except for that Spanish version of Girlfriend by Avril Lavigne. How in the world did that get in there?).
I think Dominator would be awesome if it just had Crash mode. I mean, they even have the 1 PSP multiplayer feature, the perfect nod to Crash mode. But noooo...
...Anyway...
I can't comment on the GTA games, as none of the GTA series have been my cup of tea.