The "better servo" would depend a little on tires. Larger tires would be better served with the 645mg servo, as it has more torque. The 625 is a bit faster though, and would be better IMO on smaller tires, such as panther komodos.
The 945 is the next step up, which offers the speed like the 625 and the torque like the 645. Then, the digital servos are the next step. I will have some 945mg servos listed this weekend. The digital servos(hitec) have been out of stock for a while from my distributor.