I don't know about the Aquacal unit , but heat pumps should work for any temperature range you're willing to stand around outside in a wet bathing suit. Other than that, heat pumps cost more up front but are cheaper to operate.

I have a variable speed pump, but, they are expensive and more susceptible to power quality issues than standard motors. A properly sized 2-speed pump will also provide electrical efficiency at a lower price and greater indifference to power quality problems.

Sorry, I just now got to your post. What did you decide?