The first thing to remember is that you're not choosing the iPhone for the voice plan. It wouldn't matter, either, because the AT&T and Verizon voice plans are basically identical: $40 for 450 minutes, $60 for 900 minutes and $70 for unlimited minutes.
So let's get to the meat of the argument: data plans. AT&T made the move to tiered data plans last year. That means there's a 2GB data cap for new users. The 2GB plan costs $25 per month, and AT&T also has an option to pay $15 for a paltry 200MB.
Here are some things to consider. If you know you consistently use less than 2GB per month (occasional e-mail use with no attachments, maybe a few tweets, light browsing, etc.), then you might as well save $5 with the AT&T plan. But if you use 2GB or more (streaming video and music, heavy browsing), the Verizon plan is worth the money. Even if you only occasionally go over 2GB, the Verizon unlimited data option may still be worth it because AT&T charges $10 per gigabyte over the monthly allowance.
If you don't know how much data you use, check your online account or text *DATA# (for AT&T users) or #DATA (for Verizon users).
Fortunately for those who don't currently have a data plan, both carriers have data calculators (AT&T's calculator and Verizon's calculator) that can help you determine how much data you would use in a given month.
The other thing to consider when looking at iPhone data plans is whether or not you will be tethering your iPhone. Tethering allows users to create a Wi-Fi hot spot with the iPhone, allowing Wi-Fi-enabled devices to connect to the Internet through the iPhone's 3G connection. This is usually done with a cord or Bluetooth, but the Verizon iPhone will be able to do it wirelessly with up to five devices at once (it's expected that the AT&T iPhone will receive this feature later with an update to the iOS operating system).
Tethering costs extra, and both carriers charge $20 per month; however, it's handled differently. This time it's Verizon that institutes a data cap (2GB) while AT&T has none. And Verizon charges a whopping $20 per gigabyte over the data cap.
Finally, we have the part of phone plans that has become almost trivial for smartphone users: texting. Both carriers have a $20 unlimited texts option. AT&T also has the choice of $10 for 1,000 messages, while Verizon breaks it down into $10 for 500 messages and $5 for 250 messages. Both services offer a $90 per month unlimited text and talk plan.
So here's the bottom line: If you're just texting and talking, it's a toss-up, maybe leaning toward AT&T's 200MB data option if you really only use a tiny bit of data. If you're a standard user, you may be able to stay within the 2GB AT&T data cap, but the Verizon unlimited iPhone data option sure is attractive (you'll inevitably end up using more data than you think once you discover the wonder of iPhone apps). Finally, if you're tethering, it's about even unless you plan to use more than 2GB per month (which the average user would likely burn through in no time).