What are the most important features of a FFP for you?
Keep in mind that you do not necessarily have to credit your trips to the FFP of the airline on which you will be flying. For example, you could be flying on AA, and elect to credit your flights to BA, or vice versa.
If your ultimate goal is a Business Class award ticket to the U.K., you probably don't want to accumulate BA miles because of the hefty cash surcharges collected by BA on TATL award flights.