I have had Verizon since the first cell phone from them was offered and I have had pretty much EVERY smartphone in their line up between Blackberry, Android, and now the iPhone. I did have the Thunderbolt which let you use data and talk at the same time and that was the phone that I had before the iPhone. Personally, talk and data at the same time doesn't bother me at all if I have it or not.
Yes, GSM networks are better in certain respects but the coverage base of Verizon in the US is unbeatable. My friend has a BB on AT&T and he always and I mean ALWAYS complains of dropped calls and crappy service.
Apple isn't stupid they wouldn't change the iPhone internals just to have a difference between the two carriers, AT&T and Verizon have too many customers for Apple to do something stupid like that. Its the same phone, what it comes down to is your service, you have crappy coverage by one, go for the other simple as that.
Verizon has been good to me, yes I have had my issues with them at points but who hasn't had issues with any of the phone companies? Phones are disposable I am prime example of that since I usually switch mine 2-3 times a year its the service that you keep and that binds your choices.