[lincpimp]

If you gear so that the motor produces the same output (wattage) on both 4 and 6 cells you should produce the same top speed. The higher voltage setup will be more effecient, due to the higher voltage drawing less current (amps) to produce the same output (volts x amps = watts), in turn the lower draw produces less heat in the wires, motors, esc, etc. Small gain but still there, plus stuff runs cooler, so it should last longer too...