Re: clearwire not a competitor
T-Mo has been transitionoing from HSPA+ to LTE for the last year or two. HSPA bought it some time and kept it competitive. Plus the fallout of the failed AT&T merger means T-Mobile has some access to AT&T equipment. In simple terms, T-Mobile's LTE concentrates on Band IV, AT&T's on Band XVII. Most LTE Phones sold in the US focus on those bands as well as Bands I and/or VII (which are common bands abroad). Unfortunately, US phones can't use Band III (the de facto common global band) because it's an active military frequency.
The current issue with Sprint is that, not only do they still use CDMA for voice comms and as a fallback (VoLTE is not ready yet), but their LTE bands are different (Sprint uses Bands XXV, XXVI, and thanks to Clearwire, XLI, I think) and not on more typical bands. Meaning Sprint LTE phones run the risk of not supporting the AT&T/T-Mobile bands or dropping support for international LTE bands like I and VII.