Both Bell and AT&T have the same radio frequency. So that shouldn't matter, as Bell also has AT&T '4G'.
Are you in Canada log1c? Do you think you would be likely to root your phone in the future? Bell currently has NO updates for the Attrix. AT&T already sent out 2 and a third, Gingerbread, is being tested.
Bell Atrix comes as stock as possible, i.e. not as much restriction, compared to AT&T. Tether and download from unknown sources are both available to you on the Bell model (they are taken out for AT&T).
I ask about rooting since if you are into that then the AT&T is the way to go as the Rooting community is much bigger for AT&T Atrix.
I got the Bell version because I wanted my one year warranty. I think for the AT&T one, you need to mail it to the USA Motorola for warranty claims.
So what you should consider between the two
1)Updates - AT&T wins
2)Bloatwares/restrictions - Bell wins
3)Root community - AT&T wins
4)warranty - ???