Reliable in what way? Right now Sprint LTE is not the pinnacle of reliability and it shouldn't be yet, they're still bringing the network up. Conditions are still spotty in a lot of the cities where there is actually an LTE deployment, some days there's LTE on one street corner and the next day it's not there at all. That's not to say it won't eventually be solid, but the LTE side of the network is quite scarce at the moment and reporting has been that it is fairly unstable in terms of when and where signal is received. (as per the chatter on S4GRU).
Sprint by way of the wide amount of spectrum they control through Clear might eventually be the fastest carrier, actually, but that is some time out. As it stands now, if you want to compare T-Mobile HSPA+42 with Sprint LTE under equal loading, they should perform about the same. 10 MHz SISO vs 5 MHz MIMO, both optimally 64 QAM.