What are "Watt hours"?

To work out the Watt Hours of a battery, multiply the volts times the Ampere Hour rating - eg, 36V x 10Ah = 360 watt hours.  24V x 12Ah = 288 watt hours.  Don't fall for just comparing Ampere Hours (Ah) - in this case, the 10Ah battery has higher Watt Hour (Wh) capacity and will get you further than the 12Ah battery.

A 36Volt battery and motor will also provide more torque and help you go up steeper hills, but not go faster on the straight and level.