I prefer use 2 cells lipo pack because it make less work to the regulator to down the voltage to 5.5 or 6 V when nominal is 7.4V (8.4V full charge).

Work well on nearly all my planes now (and all the bigger). My tow plane have 5 hours of possible flight (demonstrated) with only 2x 2cells 2200mAh Lipo. Engine Zenoah ZG62, airplane 102in span, 10.5kg without petrol.

In fact with 3 cells most of regulators will deliver less amperes to the servo than with 2 cells : read carefully the specs.