Range doesn't come from their system. It comes ftom you, where and how you ride. Turn the power down and pedal hard on flat roads to achieve 100 miles range. Put a fat bloke on it and ride it up the Welsh mountains, and you'll struggle for 10 miles., regardless of which version you have.
The 750w one has a 25A controller and the 500w one an 18A one, so with everything at maximum, which rarely happens, the 750w one will go for 72% of the time the 500w one will go, but it'll be going a bit faster.