ORIGINAL: krayzc-RCU
5 cell being a 6v would give you about 12-20 % more power on the servo along with having an extra cell if one goes out
It will also give the servo a bit more speed and accuracy under load. The cost will be the weight of 1 extra cell and about 7-10% less duration. I run 6 volts on all my nitro planes. There are some receivers / servos that don't like 6 volts.