Another caveat, because of the frequency that Sprint uses, I understand that reception inside buildings is somewhat worse than the lower frequencies of GSM. This was corroborated with my experience with Sprint, but I only have anecdotal evidence.
T-Mobile (GSM) uses the same frequency as Sprint. (1900 mHz)
Really, I think the frequency thing is way over-hyped. How well a network is implemented and where the towers are placed are probably far more important than what frequency it uses.
I use Sprint and generally have very good coverage inside buildings. I can even hold a call in my company's elevator.