I believe T-Mobile is bigger in Europe than Cingular, but it seems that most people using the phone internationally buy a local SIM. That's all I know from reading forums. For an extra $20/mo Cingular includes international roaming for data with the BB, not sure what T-Mobile offers. You might want to check at www.howardforums.com
As for NYC, I used T-Mobile for four years and I'm about a year on Cingular for voice. Hands down, in my experience, T-Mobile is better. Not a little better, a lot better. I use about 2200 min/mo with both carriers. Rarely does a day go by with Cingular where I either drop a call or get a fast busy (call failed) message. It is more of an annoyance than a problem. With T-Mobile, it is more like once a month.
T-Mobile has a lousy selection of phones, Cingular received the new BB six months before T-Mobile. Cingular is very slow to release software upgrades and is the only national carrier in the USA to have users on BWC rather than the newer BIS; T-Mobile had it back in October. Cingular lags far behind Verizon in 3g cell data, T-Mobile is even farther behind Cingular. T-Mobile has excellent customer service, Cingular is generally towards the bottom. T-Mobile is significantly less expensive, you can get a 3000 min/mo regional plan $50 + BB w/o BES for $20 for a total of $70/mo, get a calling card for those long distance calls. Cingular runs considerably more expensive. If you're in a city or major highway, T-Mobile has coverage but if you need service small towns Cingular has a much bigger network.