By all accounts, making a phone call on the Verizon iPhone is a far better experience than doing so on AT&T's iPhone. Clearer and fewer dropped calls are the norm. But when it comes to data, Verizon isn't quite as good.
Firstly, data speeds on Verizon have been reported to be significantly slower than AT&T's. Furthermore, if you've got an unlimited data plan, if you start using up too much data, they'll start slowing your connection down even more.
So I guess it comes down to this: what's more important to you, voice or data?