A higher voltage pack (6V)doesn't mean longer run time, just that the servos operate faster. The low-voltage limit of about 1.1V/cell is the same, so little is gained by going with an extra cellIf you need faster servos, by all means go with the added cell. If you want longer duration, add milliamp-hours.

Far better to carry the weight of a fatter 4-cell. IOW, a 4-cell 2200 pack lasts about the same as a 5-cell 2200, the 5-cell setup has faster servos. Voltage is the size of the pipe, milliamps is the size of the container.