Heat Pumps can be purchased that actually will run bellow the freeezing temperature! ( -5C or 23F)

Heat pumps are so much more efficient that any other heating method ( except environmental like solar of course).

Check the internet for Heat pump vs gas heaters
there is tons of info on comparing the two.
( I saw a gov't study that showed that a heat pump could pay for itself in savings over a three seasons period compared to other methods... again, not including solar)

True, a heat pump isn't as fast as gas, but how fast do you need it to be?

I turned my summit heat pump ( 115000 btu) on at a water temp of about 54F... the air temp did not go above 65F ( most of the time way less), rained every night, didn't have a cover on my pool, & in less then a week the 140000L ( 37K gallon?) pool was 90F

It maintains heat wonderfully as well

To me, it isn't even a question:
If you want efficiency, you go with a heat pump

next question, go with a titanium copeland scroll ?.... (lol)