Ok, I was under the impression it was the opposite. But I wasn't able to find much material explicitly discussing the differences (pros/cons) of AT&T's 3G network vs T-Mobile's.

Guess I should also add that AT&T's 3G is compatible with Telus 3G in Canada... at least according to an article that I read, the Droid/Motorola Milestone that will be sold for Telus will only be compatible with AT&T 3G, and not T-Mobile.