at&t iphone 5 = GSM only. it can be used on any GSM network worldwide (once unlocked) but NOT on any CDMA network
verizon iphone 5 = CDMA and GSM. it can be used ONLY on verizon, and then any other GSM network in the world (once unlocked)
basically, both the verizon and at&t phone are compatible with any GSM network, but only the verizon one can be used on verizon. if you switch to verizon from at&t, you will need to sell your current phone and buy a verizon one.