it's not a new game, getting as many players (paid or froob) in the beta is gonna cut down on how long it needs beta testing. the whole point of beta testing is to find all the bugs and issues, the more people testing the better.
limiting it to current paid, or loyal players is a bad idea, I doubt there are enough paid players that would beta test. allowing all current paid and froob in the beta will provide the most diverse hardware array for testing.