Played and finished both.
I liked Colosseum more because of Wes & Rui and 'cause you had both Espeon and Umbreon from the start while in XD you could only have one.
Also because of availability of legendary beasts as they could have been used in game storyline.
This was missing in XD where XD001 and legendary birds were at possession of the last boss so you couldn't pretty much use them... :/
But XD001 was the main reason why I liked XD as well but Colosseum is better imo.