The actual data plans att sell are the same it just depends how much data you use and how much they charge you. It is a money thing it is not technology related.
Lets say I buy the new apple phone on a contract or I buy the same exact phone at full price with no contract. In both cases I will get a sim card that will allow me to connect to att network. They will connect to exactly the same data towers and work exactly the same.
So you have 2 things to decide on. First you must find hardware you like that is compatible with ATT network. They sell prepaid phones that can run the latest LTE or they sell basic ones. Of course any phone can be a "prepaid" phone if you pay full price for it.
Then you must decide on a plan. You can put prepaid phones on a contract plan if you like just like you can put contract phones on prepaid. The plan is mostly based on how much data you expect to consume.
Coverage is a completely different issue. It is not related to how you pay for things. So your first step is to find out which vendor has the best coverage in your area and then try to find a device that you like that is compatible with their system. Verizon tends to have the most extensive data coverage with att close behind. Sprint and tmobile have must less coverage but tend to be cheaper.
Almost all the other carriers you find other than a small handful are actually buying service from one of the large vendors and reselling it under their name.