You can use both T-Mobile and AT&T, but there are some areas that will only have 850mhz. At least for 3g most US markets on AT&T are going dual band and there is very good coverage for either.
You can blame this fractionalization on the way that the Us handled frequency allocations. 850 is the preferred spectrum since it covers a bigger footprint per site. My suggestion is to get your 1900 handset and then buy an ATT (or Tmobile) pay-as-you-go phone which you can get REALLY cheap. (as in $20)
That gives you a backup if you are going through an area that has no coverage for 1900.