On motor power, note that the power rating of a motor is usually the maximum continuous power the motor is rated for, ie without overheating. Just because a motor is 2000W as opposed to 600W, doesn't necessarily mean it will use any more power for a given job it is being asked to do. Your 600W motor could produce 2000W too if you were to nearly double the voltage, but it might overheat. Regardless, the power they draw is the power asked of them. For example, your skateboard might be 200W continuous when travelling at a constant velocity, and both motors in this case will only use 200W, or rather a bit more because they're not perfectly efficient. Which brings me on to efficiency. Different kinds of electric motors have different characteristics, but it's usually the case that they will have a maximum efficiency at a lower power than maximum power. The designers of this skateboard might have chosen their 2kW motor such that it is operating at maximum efficiency at the power they are expecting it to need to produce to propel humans around. In this respect the range of the 2kW skateboard might actually be greater than the 600W skateboard, everything else being equal.

That said, 2kW is a lot of power! If all that could be instantaneously put down without wheel spin by a skateboard, I image it would shoot out from under your legs like a bullet out of a gun. Fun!