If you really want to save money, prepay is the way to go.
With that said, I'm currently under the 700min family plan with AT&T and the bills are around $90 for 3 lines.
So, if you drop it down to 2 lines and get the 500min plan, it would be $20 bucks cheaper, around $70.
The thing about AT&T roll over plan is that, any unused minutes from one paid period to another will be counted as rollover minutes.
So, let say in a particular month, you only use 400min out of 500min allotment, the unused 100min will counts as rollover. Then if in a particular month you use 600min, the overage of 100min will be docked off from your total rollover minutes and you'll not be charge extra for it. Also, the rollover minutes will expire a year from the day they rollover.
If you want to switch, I would suggest you finding a friend that has AT&T and test out their network to see if you like it or not. If you like it, the next thing to do is to find a deal for phone. Usually different places offer different deals. For example, when I was going to switch from Verizon to AT&T, I went to an AT&T store at a mall near me and they offered free low end basic phone when you sign up for 2 line or more and only for the first 2 line will you get the free phone.
A few paces from where the AT&t store is, there's a RadioShack offered $300 gift card when you sign up for 3 lines, $150 for 2 line and $50 for 1 line, and they offered free phone for up to 3 lines. The phone they offered were not low end basic phone either, I got Moto razr v8 or something like that with retail price of $250. I jumped on the deal and got the phone, sold it on ebay for $200 and got a Sony K790a instead.