Definitely check out the Aquacal SQ150VS unit. This is a heat/cool heat pump, but .... it uses a variable speed inverter driven scroll compressor. It also has a titanium heat exchanger. It is the only variable speed heat pump I could find which is designed for pools. Variable speed home heat pumps are extremely common now, due to their high efficiency and low operating cost ... but it's taken a while for the pool industry to catch on/up.
The variable speed inverter drive means it is VERY efficient in maintaining the water temp. I installed this model back in Feb/mar? and have been extremely happy with it. After I got it installed, we heated our 14k pool to 88* (from low 70's) and it added about $20 to our electric bill

I then maintained that temp over the weekend, and the heat pump happily ran in low speed, costing me pennies.