It's still early and not all the details have been revealed, but the early indications suggest that Verizon may have the upper hand on AT&T for iPhone fans who can afford to get out of their contracts and for those yet to experience the joy and wonder called iPhone.
Data plan details aren't completely certain, but a slightly faster experience on the Verizon network along with a different antennae design and better customer experience ratings may ultimately mean a more reliable overall phone. International use will be limited given the Verizon GSM network and some network-dependent multi-tasking may also be limited with Verizon where talking and data use are needed at the same time.
The big deal with the Verizon phone will be the use of the iPhone as a portable hotspot allowing you to use the phone as a wireless access point for other devices like your laptop or iPad or for benevolent sharing with other folks (or a significant other) who need wireless access.
Where AT&T data plans are restricted to 2GB for heavy users, Verizon indications are that unlimited data plans will truly be unlimited but there are still questions about cost and tethering restrictions on data if you decide not to use the hotspot functionality.
It's a close call, but for now, I think Verizon wins on data and hotspot functionality allowing users to truly take full advantage of their phones and save some cash without the need for additional internet access devices or pay-as-you-go data plans.