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XD had a better variety of obtainable Pokemon, and they were actually usable since there was more than one Shadow move.
Additionally, XD has the better graphics (particularly the character models).
That said, Colosseum's story was more interesting due to the MC being a former Snagem Admin, and there was an actual postgame as well.
A lot comes down to personal taste, so while I prefer XD, they're both very good.
