The answer totally depends on where you live, but I will say this:
I live in NJ and I've had extensive experience with both T-Mobile and Cingular, as I bounce back and forth taking advantage of their "new customer" promotions. In my honest opinion I would stay the HELL away from T-Mobile. After having a ton of dropped calls and NO SERVICE AT ALL in my apartment I have to say their service is subpar.
Both my sister and I have had nothing but trouble with T-Mobile's customer service. She had a phone stop working and went through 6 months of hassle obtaining a replacement (2, actually) to fix the problem and THEN got slammed with a "not-returned fee" for one phone that T-Mobile lost. It took over a dozen phone calls to T-Mobile customer service, phone calls to the U.S. Post Office distribution centers in multiple states, and finally a letter AND a followup phone call to the Vice President of corporate customer service to get her $180 credited back to her.
Similarly, I lost a cell phone and decided to purchase another one from them. While it was being shipped I had second thoughts and upon receipt refused the delivery (on their advice). This time they did in fact credit me for the phone but slapped me with an early termination fee because the customer service rep decided to "upgrade" my account and tack on another year of service. And believe me I repeatedly told him I did not want to upgrade, I would rather pay full price for the phone than $60 a month for another year to save like $90 on the purchase price of the phone. I cancelled with T-Mobile this past November and STILL have not received my $200 credit forwarded to me as a check. If they do not ship it to me by the end of March, I will be filing with small claims court to force them to pay.
On the flipside I have never once had any problems with Cingular. In fact when I switched over they incorrectly signed me up on a lesser phone plan than I wanted (a simple mistake) and upon receiving a $240 phone bill for my first month of service, it took only one call to rectify the error and wipe the $240 from my account. Cingular service never drops out and I rarely have little or no reception. Their service is just better in NJ.