I noticed some minor differences between the two Curve 8900s that each carriers sells. First, the keyboards are different in their superficial looks - The At&t 8900 has a keyboard that resembles the 83xx more than the T-mobile 8900. And correct me if i'm wrong, but the At&t has a black battery door, which looks better than the "titanium" of T-mobile's. Overall I think the At&t one looks better.
I carry the T-Mobile build but sell the AT&T build. The T-Mobile build looks better. (I hate the AT&T build's keypad.) However, scratches on the back of the T-Mobile ones show SO much more than identical scratches on the AT&T ones.
Here's another vote for the AT&T keypad. Also, isn't the T-Mo trim more like a nickel color versus the bright chrome of the AT&T version? Maybe my eyes are playing tricks on me but I did prefer the more muted T-Mo version.
I prefer the TMo but I also just recently changed out the bezel to black since the chrome was getting so scratched up from normal wear. It looks pretty sweet in all black with the red number keys. Just my opinion.