I disagree with BasicGreatGuy. The only way you can save money by going with Verizon is if you don't get AppleCare. Both programs are 0% interest, but the IUP includes AppleCare in the financed cost. Quite frankly, IMO, AppleCare should be a requirement as it's the cheapest insurance option out there.
The argument against going with a carrier finance plan like Verizon is if you want to upgrade after a year, you HAVE to stay with Verizon. With the IUP, you can go with any carrier you want at any time. Including during the finance period.
The only stipulation in the terms and conditions of the IUP referencing 6 months is the ability to upgrade beginning at 6 months, not 1 year, if at the time you make the equivalent of 6 more months to equal 12 months of payments. In a way, this can be an advantage for the IUP in that if you want a new phone after 6 months, you have the flexibility to do so. I haven't seen anything stating a penalty if you decide to pay your loan off early. Given that it's a 0% interest loan, not quite sure what they would get out of charging you one.
As for the credit hits, nothing says Verizon doesn't conduct periodic credit reviews and won't do so if you want to upgrade your phone. In fact, if you read many of your credit agreements, most all of them will say they have the right to periodically review your credit for one reason or another. I'll also say that if a credit check for $1000 or less will affect your credit rating, you have other more pressing issues to be concerned about than financing a phone.
Edit - if you are currently with AT&T and have two phones in the IUP, you don't have to get new phones. Apple has a program to replace your AT&T phone with one that will work on Verizon or Sprint. A thread is active on the forums addressing this and should be easy to find. It's a new program that has some snags but word is getting out.