If you're also getting a quadband phone, then you're fine. And since you're switching over to T-mobile you'll be fine as well. Quadband on Cingular will give you better reception because they run on the 850 band. Also, if you travel in North America, it's always good to have a quadband phone that way you'll have full coverage no matter where you go. I know T-mobile have some quadband phones for that reason.
Go for the n91 since you'll be switching over to T-MO. I have to admit the build quality on that phone is darn nice! Very solid phone. It's too bad Nokia didn't come out with a quadband version or atleast an 850 version.