VS pumps are VERY expensive and, from, what we gather, aren't much more efficient than far less expensive 2-speed pumps. Plus if they break they are harder and more expensive to fix. I've been VERY happy with a 2-speed pump, 1 HP for a 19-20,000 gallon pool, with solar panels and a 200 lb sand filter.

Beyond that, pump size must be matched to filter size (too big a pump is a problem as it damages the filter). Both should be matched to the pool size, as well as any water effects you have (water falls, slides, heaters, solar panels, hot tub, etc). Then there's the pump's planned location re: the pool, all of which figures in.

But one thing: Pump horsepower isn't like car or outboard motor horsepower. More isn't necessarily better, but in fact can be very expensive to run.