6 volts= faster and stronger servos BUT unless you really need the extra speed or strength, you will never notice a difference.
On the negative side, a higher voltage battery of the same capacity will not run as long due to the higher current draw by the servos.
On the plus side, a 6 volt battery may be safer if a cell is lost in flight, This is only if the lost cell shorts and can still pass current from the remaining 4 cells.
Cost, of course, is higher on the 5 cell pack as is weight.