For all JR/Spektrum radios I'd prefer a large capacity 4 cell or better yet a 5 cell due to the Rx cutoff voltage being higher than the others. For non JR/Spektrum radios the 600MAH pack is a good starting point but with digital servos I'd increase capacity and stick to 4 cells because you wouldn't need to change chargers (just charge longer if fully discharged). WHATEVER you choose be sure to check them before every flight.