Heh. You'd better visit those stores more often.
Anyways, yes, XD IS a sequal to Colosseum. Nearly all features of Colosseum are in XD, except for the lvl. 50 or 100 single-player colosseum battles. (only one in-game, and those Pok?mon match all lvls. 60 or higher) That means more catching of Shadow Pok?mon, over twice as much.
But do please post this in the Simple Questions thread next time. Unless you want to make a discussion of a topic, please post there.