The 5 is clearly going to have a longer useful life. That and the fact that most carriers require a 2 year commitment to lock in the price means that what you buy will have to last for a couple years. I'm not sure a 4 or 4S would keep me a happy camper in a year or two. Phone tech moves really fast so consider that the 5 has a much more powerful processor and likely will be able to run most of the newer services/apps that come along.
SIRI is one example. SIRI is, for me, a must have feature. Doesn't run on the older phones. As new features come along, like SIRI, they may not run on an older phone.
The extra $100 for the 5, when spread over two years, is not all that much money, under $0.14 per day.
Another thing to keep in mind is that your daughter is growing up and before you know it she will be off on her own. Take every opportunity to make her happy while she is still at home.
I'm on AT&T and my plan rolled over with no issues when I added the 5. I didn't loose the plan I'm on even though it is no longer offered by ATT. I was grandfathered.
You might ask to speak with a senior manager at your carrier and see if you can get them to accommodate your request. Most have the authority to make exceptions, especially in that you are locking in a long term revenue source to them.
