Postpaid plan for 2 iPhones: $170/month
Prepaid plan for 2 iPhones: $98/month
Frankly, the math was quite simple.
I decided to go with Verizon's Prepaid service, mainly because our existing phones were iPhone 5's, Verizon flavor. So instead of buying new phones, we just bought the SIM card kits. If I was also looking to buy a phone I would also consider the other prepaid services available (StraightTalk, Cricket, Boost Mobile, TracFone, T-Mobile, Virgin).