It depends on how quickly you want your phone, what your travel patterns are, and what the coverage in your area is. Both AT&T and VZW are pretty comparable with price. Verizon is way ahead in LTE deployment nationally and will have their whole network deployed within the next year, so you will be able to get LTE pretty much everywhere in the US. I think AT&T's deployment on this is 2-3 years away. Verizon also has the spectrum to ramp up LTE to its full 300 Mbit/s top end and will begin jacking the speed up on it after the network is fully deployed. So chances are good that the speed will increase.
I have it on good authority that VZW will be getting a variant of the 920. I have heard that it would be out for Black Friday, though that could always change. But VZW seems to have plans to carry the 822, the 92(2?), the HTC 8X, the Samsung ATIV S, and, maybe, the 8S too. But don't expect them all to hit in early November. Verizon likes to stagger phone releases, so I'd guess they'll have two WP8 handsets available first week of November (likely 8X and 822), then they'll add the ATIV S, then they'll add the 920 variant.
I have heard nothing other than AT&T threw some money at Nokia to get the 920 exclusive, but they care mostly about it because of the buzz surrounding the number. Verizon's will have a modified design and one bit of info I've heard is that it might add microSD support, but that is only buzz at this point.