Ehh, I'd stay with Sprint, as crazy as it sounds. It sucks right now with my 4S, but when I upgrade to the 5S I'll have unlimited LTE 800/1900.
Sprint - Unlimited, decent coverage, variable data speeds (3G is horrendous) but the network is improving a lot.
T-Mobile - Decent plans, unlimited (I believe), great data speeds, but poor building penetration and lousy service outside of urban/suburban areas.
AT&T - Overpriced, limited, great data speeds (now), but spotty/poor coverage where I live. I used to have to stand by the window to use data or make calls. I left for a reason.
Verizon - Overpriced, limited, good coverage overall, with variable data speeds in NYC. No thanks.