First of all, if the two motors come from two different manufacturers or sellers the watt specs may not compare at all. Often, 500 watt and 700 watt motors are very similar motors able to take 1000 watts, but with different controllers. One may be an 18 amp controller and the other a 22 amp, or something like that. In other cases, one seller rates continuous wattage and the other rates peak wattage. In fact, it may be the same dang kit.
But to better answer your question, I'll rephrase it. Same motor, run at different wattages. It could be big and small controllers, or simply using less throttle on the same kit compared to full throttle. Both achieve the same thing, less watts vs more.
The lower wattage will not climb hills as well, but may not use that much less to get up the hill. Climbing simply takes a certain work to lift a certain weight up the hill, period. My experience with hills is that climbing at 5 mph is not that much more efficient than climbing at 10 mph. Slow may even turn more of the power into heat, since you are lugging a direct drive motor at speeds under 12 mph. In either case, the hill is never less work to get up it.
But on the flatland, slower is allways going to go the furthest. The wind drag is the main reason. every mph more really costs you. It's exponential, so 10 mph is way way way more efficient than 20. For me, I have a motor that goes 20 miles at 25 mph. At 12 mph I can go almost 40. 16 mph I can go about 30, so I find 16-18 mph a good speed when I need more range. Not unbearably slow, but 33% more range. So clearly the lower wattage will go further if the result is a slower speed. With wind drag, there is a way to lessen the work, and less drag means more range.
Rephrasing the question again, I have tested another motor that was a gearmotor using 350 watts vs a direct drive motor using 700 watts. When I slowed down some , to about the same max speed as the gearmotor, I got really similar ranges on both. I suppose the two motors at the same 20 mph speed were actually using very similar wattages. The interesting thing was that very different motors used about the same power to go the same speed. The reason for this has to do with the wattage needed by the motor at cruising speeds. The 350 watt motor at it's max watts could cruise at 20 mph, while the 700 watt motor needed only 350 of it's avaliable 700 watts to cruise at 20 mph. So in a way, both were 350 watt motors when cruising at 20 mph.
