There are a lot of variables involved, but in general a 48v motor is going to spin faster and use more current (amps) and require a larger more expensive battery than a 36v. 48v is 33% more power than a 36v (includes more weight, expense, size etc.)

I like small and light, after all I can ride a bike without a motor and don't mind the exercise.

Battery is really your most important item (size, weight, cost, happiness). Many of the motors (within a brand) are basically the same, just geared different for different speeds based on the battery. The big difference between these motors is the controller. the 1000 watt motors are going to have a controller that uses more current, and that is going to require a bigger battery.