I think it will depend on where you live. Sprint seemingly tends to be better in the Eastern United States, T-Mobile in the Western US. Both have somewhat spotty coverage, it often depends on which works best in your city/neighborhood.
I also had Virgin Mobile and quit when my data speeds were frequently dial up speeds, and typically were not faster than 300 Kbps. Also, WiMax, while in my city, did not work in the neighborhoods that I need it. My T-Mobile experience has been quite good; I've particularly enjoyed getting similar speeds to WiMax in most areas of my city, even the neighborhoods where WiMax wasn't working -- and getting them without having to turn on a separate antenna.