At the end of a 2-year contract, you can get AT&T to unlock your iPhones, which will allow you to put any SIM card you want in for phone service. That's probably the best option.
Straight Talk resells AT&T service for $45/month unlimited, and has other plans:https://www.straighttalk.com/secure/ServicePlans
T-Mobile sells its own service for $50/mo unlimited, and less for plans with either less data or fewer minutes:http://prepaid-phones.t-mobile.com/monthly-4g-plans
They are currently upgrading their service so iPhones can use 3G:http://gigaom.com/apple/nyc-and-13-more-cities-get-t-mobiles-iphone-friendly-upgrade/
If you are in other markets, iPhones can only get EDGE (low-speed) web for now.
The other great thing about prepaid service is there's no risk of huge bills for overages. Instead, you get charged a reasonable rate by-the-minute, or service is cut off until you put more money into the account.
Once smart phones are smart enough to have at least a one-week battery life after normal use.
My Motorola V195 does about 2 weeks if I have good reception.
"Normal use" means something entirely different on a smartphone, which has a huge screen that it regularly has to light up for minutes or hours on end to display useful information it has to download and process at high speeds, all of which require a nontrivial amount of wattage. If you use a smartphone only as a telephone, its battery would last much longer, since the cellular modem would be seeing relatively lightweight usage, and the screen and computer chips would be shut down most of the time. But most people with a smartphone end up using its non-phone features a lot, because they're so dang useful.
If you want a top-of-the-line smartphone on a budget, buy an unlocked Nexus from Google for $300 (https://play.google.com/store/devices/details?id=nexus_4_8gb
) and whatever GSM plan makes the most sense for you. I pay $30 to T-Mobile for prepaid service.