The Pros of each:
Cingular:
Better nationwide coverage in more rural areas and smaller cities/towns
Rollover Minutes (usable up to 1 year)
Better nationwide coverage for wireless data plans (Edge and HSDPA)
Large selection of phones including many new ones like the iPhone
Walkie Talkie (PTT) Service available in USA on select phones
T-Mobile:
Good coverage if you remain in larger cities and on interstate highways
More anytime minutes for the money (ie individual voice plans)
No extra monthly charge to add Int'l roaming
1 year contract available
Some claim better voice call quality
Based on recent experience who drops more calls: Cingular or T-Mobile?
Excellent summary. I would add:
Customer service: T-mobile
Unlock phone: T-mobile (unless Cingular/ATT has relaxed their policy)
Coverage when in subways, trains and tunnels: Cingular