cdesja you are partially incorrect about flexability in the us. 2/3rds of the cell phone market uses CDMA here without swappable sim card (verizon/sprint/uscellular/metropcs,cricket) all use CDMA only At&T and Tmobile have GSM phones with interchangable SIMS. SOME of the carriers with CDMA have LTE phones that can use a european GSM sim but will not work with american GSM chip.
There really isnt much flexability in american cell phone options unless you go pre-paid. There are several things in place that american carriers have that deter moving to different carriers, one is forced incompatability with other phones of the same technology, large ETF's, 2 year Contracts and phone exclusivity deals.