The power ratings are mostly for marketing purposes. Actual power consumption is based on how much the controller draws. The SP controller lists 18 amps max continuous (which is around 860 watts). Max burst you can set it to is around 30 or 35 I think, but I've never seen it draw more than 20 on my bike. Cruising you use a lot less. This is mostly when accelerating and going up steep hills. The SP gets pretty inefficient at high power anyways because it doesnt have the same pulling power of an MP. Youre basically just burning energy if you try to draw more amps. Think about it like leverage on a wrench. A smaller wrench doesnt give you the same leverage a larger one does when unscrewing a bolt. MP accelerates faster and climbs hills better. Top speed is about the same. Since its a direct drive (one "speed"), your top speed will be more controlled by the diameter of your wheel.
All that being said, I've found the SP is great for cycling and meets all my needs. Unless you live in a very hilly area, an SP should suffice. I can easily cruise at 25 mph with just some light pedalling. If youre looking to pop wheelies or climb the hills of San Fran, then go MP. The SP might be a little more efficient since it weighs less, but it wont make a huge difference in wh/mile.