The important thing is to first determine which major carriers have the best/strongest coverage in your geo area and also areas where you travel. Your user experience will depend on the quality of the signal you get. All the mobile vendors use one or more of the underlying big cell companies and they resell the service.
So for instance:
Virgin Mobile uses Sprint (Sprint is quite weak in my area)
Boost Mobile uses Sprint and Clearwire (for WiMax)
Straight talk (a division of Tracfone) uses different companies depending on location. In my area they use AT&T and T-Mobile. T-mobile is weak in my geo area and AT&T is stronger.
Here's a list of the pre-paid MVNOs
List of United States mobile virtual network operators - Wikipedia, the free encyclopedia