Here is what you need to know.
Want to surf while you talk? Go with AT&T
If you go with Verizon, you cannot use the internet while you are on the phone. On AT&T, you can talk and surf all day. That might sound like a non-factor, but it xomes in handy. There are plenty of times when I am on the phone with Sam or opposing counsel and I need to look up a document in Dropbox. Is it completely necessary? No. Is it helpful? Very.
Along the same lines, talking while referencing an email is also helpful. In theory, if you use the Mail app for your work email, you might be able to look at email offline while talking on the phone. That theory, however, has not been tested.
Bottom line: if you want to surf and talk, you can only do it with AT&T.
Need to create a wireless hotspot? Go with Verizon
The Verizon iPhone will allow you to create a 3G hotspot for up to 5 other devices, which is pretty sweet. AT&T offers tethering for one device, but it is an additional expense, and you can only tether to one device, not multiple devices.
I do not use tethering. It would be neat to have it, but I can do almost everything lawyerly on my iPhone, so I am not interested in paying for it. If it were free, however, I am sure I would use it. Given how much data iPhones suck down though, I have a feeling the hotspot option could quickly become a paid option.
As of right now, however, if hotspots are your thing, go with Verizon.
Which company is cheaper?
Complete unknown at this point—Verizon has not published any data rates. Speculating on rumors is silly at this point. Do not assume, however, that you need to pay for an unlimited or large data plan.
I use my iPhone a ton. Both my house and my office have Wi-Fi, which has limited my actual 3G data usage to always less than 2GB, and sometimes under 200MB (the two available data plans on AT&T). Keep that in mind when you are comparing plans.
Verizon allegedly has better coverage
Many AT&T iPhone users are eager to jump ship because of the alleged bad coverage. The only time I cannot use my iPhone is when I am at a sporting event. Apparently every other person is trying to check their fantasy sports team during the game, which is not a deal-breaker. Then again, not being able to call my mother-in-law, who was watching our baby, was very annoying.
Ask your friends to see what provider they use, and whether they are happy with the coverage. Both providers also have service coverage maps on their website, which is helpful, but asking your friends will provide much better information on the quality of coverage.