There is no *true* unlimited prepaid data

They all either throttle your speed, or cut you off entirely. You need a post-paid plan for real unlimited data unfortunately.

Ask yourself though, how much data do you actually NEED. For example, my wife is still grandfathered on an old ATT unlimited iPhone data plan, but she rarely goes over 1-1.5GB per month. If you aren't tethering a PC, streaming much video, etc., try to calculate what you really need. If an unlimited prepaid plan would otherwise save you money, but really only let you have 2GB as a ceiling on their "unlimited" offerings, perhaps that might suffice depending on your particular usage patterns.

Some ATT MVNOs that you might look into include H2O & Net10. Both let you bring in your own device (along with a few others), but cap @ 2GB though. Bob is actually incorrect regarding bringing in your own device. That has changed quite a lot over the last 24 mos in the marketplace.