AT&T/T-Mobile = GSM Only
Verizon/Sprint (?) = GSM + CDMA
Most people want the Verizon version because it gives the best resale value when it comes time to sell it (IE anyone on an carrier can use it).
However do NOTE that a phone tied to an account (via online means) supposedly has a mandatory 3 month lock on release before it can be "unlocked" from an account. When I originally tried to purchase my 7, they didn't have a sim-free version at release. I was going to use my moms Verizon account, however I was told that even with buying (pre-order) it outright online that it wouldn't be truly unlocked until the device accumulated 3 months of service. Dunno if that's the case now or if that's changed or if I even got the wrong information.
So on launch day I just waited in line, and purchased the Verizon version without tying it to an account.