Unless your cost of electricity is very low you'ld probably find a natural gas heater far more cost effective in your location and for your desired goal. Heat pumps are not terribly popular here in Canada as they are not effective much of the time due to overnight temperatures, and since they cost roughly the same as a large AC unit to operate (in the range of 1KW/hour) they are quite expensive to operate for long periods. In comparison, natural gas is cheap.
As for a solar cover, they're almost mandatory in the north unless you want to fight a nearly constant losing battle trying to keep the water warm, or you don't mind watching your $$$'s blow off into the wind when you get a few cool overnights. I couldn't fathom not having one, no question. Buy a good roller and they're not that much of a pain unless your pool is an odd shape, but a rectangular 20x40, well...it doesn't get much easier than that.
