The iPhone will cost the same to purchase from either carrier, $199 for the 16GB model and $299 for the 32GB model. AT&T, however, offers refurbished iPhones at $99 and $199 for the 16 and 32GB versions, respectively. Verizon won't be able to offer refurbished models when the iPhone initially goes on sale, but it is offering debit cards of varying value to those who trade in another device.
Customers wishing to avoid contracts might want to pay full retail for the iPhone 4. At AT&T, the iPhone 4 costs $599 for 16GB and $699 for 32GB (that's the unsubsidized price). At Verizon, buying the iPhone 4 at full retail costs $649 and $749. Verizon has not explained the reason for the extra $50 cost.
AT&T requires a $39.99 monthly voice plan with the iPhone 4 at a minimum, in addition to a data plan. The data plans cost $15 for 250MB or $25 for 2GB. Add tethering, and the total data package jumps to $45 per month. Neither of these fees includes text messaging. So, the least you're going to spend per month before taxes and fees (and text messaging) is $39.99 + $15 = $54.99.
Verizon Wireless has not yet announced what voice plan it will require with the iPhone. However, it requires a $39.99 voice plan for most other smartphones, so it is safe to assume that will be the minimum. Moving on to data, Verizon has nixed its entry-level data plan of $15 and will be offering a $30 unlimited data option instead. Want to add tethering? That costs $20 more, pushing to total data cost to $50 per month. As with AT&T, these plans don't include messaging, that will cost extra. So, with Verizon Wireless the least you're going to spend per month before taxes and fees (and add-ons) is $39.99 + $30= $69.99.
Keep in mind, these are absolute minimum monthly costs, and don't include messaging, taxes, fees, and any other plan add-ons that the user might choose. The $15 difference in base pricing gives Verizon customers unlimited data, however, compared to a measly 250MB over at AT&T. Monthly plans can skyrocket over $100 easily if you add unlimited messaging, insurance, and/or other services.
In my experience over the years, Verizon's network has been consistently more reliable all around the U.S., though speeds are limited. The fastest data speeds you'll get with a Verizon iPhone will probably be in the 2Mbps to 2.7Mbps range, with average data speeds falling in the 1.4Mbps to 1.8Mbps range. AT&T's network is, on average, faster than Verizon's, but I've found it to be much less reliable. With the iPhone 4 on AT&T, I've seen peak download speeds well over 5Mbps, with the average speed coming in at about 3.5 or 3.6Mbps.
To be fair, the iPhone's performance on Verizon's network is completely untested. AT&T has millions of iPhones accessing its network every day. Verizon's has none so far. No one can predict with certainty just how well the device will perform day-to-day on Verizon's network and if it will or won't have any detrimental effect on the network (but it probably won't).
One last issue to consider is portability. The AT&T iPhone 4 can be used outside the U.S. in most European, Asian, and other countries. The iPhone 4 for Verizon will be limited mostly to the U.S., Canada, and a handful of South American countries. This is due to the differences in radio technology built into each device.
How best to make a decision? The retail cost of the iPhone isn't going to vary significantly enough between the carriers to make much of a difference. The way AT&T and Verizon are pricing their services will make a bigger impact over the life of a two-year contract. It is worth sitting down and pricing out exactly what features you'll use each month, total it up, and look at the difference each month between the two carriers. Don't forget to multiply that number by 24 for the standard two-year contract.
Quality of the network is another major consideration. Do you prefer speed or reliability? Right now, there's a noticeable differentiation in speed and reliability between the two networks. Which is more important to you?
Last, be sure to weigh upgradeability. Verizon is smack in the middle of deploying a faster 4G network that the iPhone 4 currently can't use. It may intro an LTE iPhone 6 months from now, 12 months from now, etc. AT&T is pushing its network to faster speeds with LTE this summer. Given Apple's yearly refresh cycle for the iPhone, it's possible that the iPhone 5 (for AT&T) will support LTE -- or at the very least, faster HSPA+ speeds.
Yeah, I know, it's a lot. Considering the two-year cost of a smartphone, be sure to do the math and think hard.