First, they have 40Mhz there. T-Mobile also owns the D block in the NYC area. Second, yes you may notice a difference becuase Cingular may have less terrestrial lines leading to their switches which means you might get a network busy with them and not with T-Mobile which would not be dependent on how congested the actual RF interface is.
Yes, you're right about the 40Mhz. My bad. The FCC, however, lists Cingular as only sharing the A block which is 30Mhz, even though they technically share all 40Mhz (A & D blocks). Cingular had line problems in the past few months which led to circuits busy messages especially after 9PM and weekends. However, I've noticed that in the past week those problems have been disappearing. I haven't noticed system busies anymore.
Which is why people think Cingular in the city sucks. For the Average Joe a fast busy signal is just as bad as a dropped call which is just as bad as an all circuits busy.
i have gsm service with at&t here in new york city, i switched after being with t-mobile. it seems everyone has a slightly different experience with the different providers, but for me, at&t has been infinitely better than t-mobile was. call quality is much better, i get way, way fewer dropped calls and even the gprs seems faster. plus, i travel frequently to canada, and the 9.99 north american package that at&t offers is the bomb diggity, it allows me to use all my daytime and unlimited evening/weekend minutes in canada without roaming or long distance charges. i know that at&t's gsm coverage is a bit dodgy in many places, but inside new york city, as well as in long island and coastal conneticut, i've found it to be excellent.
Most people don't know this but T-Mobile limits their GPRS speed which is why it might seem slower than AT&T.
I am actualy considering getting a Go Phone from ATT to test out there network here in the city. Should be iuntresting to see. From what I here from friends both ATT and Tmobile have gotten alot better in the city. This will be a fun experiment.