XD. Definitely. Unless you're a 2nd Gen freak, XD is a much better game in every respect. Polished graphics and plot, exclussive moves, capability of getting most exclussive Pokémon in one go, hard Items and Pokémon made easy to get... just the best utility for Trainers.
I just got it, and it fixed every thing where Colosseum failed (Despite the main character and most villains sucking balls when compared with the all-awesome Wes and the Colosseum villains...). I'm loving it in every way!
I personally can say that XD is the best buy. But again, if you don't have access to some 2nd Gen Pokémon and love them too much to not have them, Colosseum might be a better choice. Plus, it has a Battle Mode a la Stadium, which XD lacks (Not that it needs it anyway.

). Your decission, but I say XD.