As far as Cingular and T-Mobile go, I don't think that they get the same exact signal everywhere. They operate on different bands, with Cingular often using the 850mhz band, while T-Mobile is solely on the 1900 band I believe. Cingular also uses the 1900 band, but in certain areas they pretty much use nothing but 850.
IMHO, I think that DubDub is correct as far as overall continental coverage with Cingular and Verizon. There are ups and downs to every company, for instance, Verizon cripples their phones. Cingular has probably the most expensive plans and internet. There are a lot of factors to throw in there when deciding. You will need to look at your best interest, where you live, your needs, etc. before choosing a network.
If I had to choose, I would go with Cingular/AT&T, then T-Mobile, then possibly a CDMA provider. I like using a SIM card and being able to switch phones out at my leisure. I like that I can purcahse a phone not available by my provider and use it when I want. I would probably choose Verizon last, even if they have the best CDMA network out there. I cannot stand that they cripple their phones the way they do. That just does not sit right with me.