I've done a lot of research on gas vs heat pumps for my pool and I thought some of my real world experience might benefit someone else.
I live in Maryland and bought my house with existing pool 2 years ago. My pool is about 30k gallons w/ hot tub. I would classify it as partial sun (direct sun from 11am - 5pm). Pool had a 400k propane Raypack heater. Heater worked great and could heat pool "relatively" quick. I would say about 1 degree an hour. Price of propane these days in my neck of the woods is about 2.20 give or take since I moved in. Without heating, in the summer, my pool would get to around 80 degree on its on. I understand that is perfect for some people, but my family grew up with a heated pool and won't go near it unless it is in the mid 80s. First year+ we used the propane heater for parties and on nice weekends we knew we were going to swim. Even then, I needed at least 4-5 hours notice (best case scenario). Just running the propane about 10 weekends a year probably cost me $2000 for the season.
This summer, I installed an Aquacal SQ175 heat pump in line with the gas heater. Heat pump / pump turns on at 2AM and usually has the temp of the pool up to 85 by about 9am and then shuts off for the day. On hot afternoons, it will get up to 87 on its own, before cooling some overnight. I do NOT use a pool cover. My estimate for my added electric bill in the summer months is about $100/month to run the heat pump to constantly keep the pool at 85. Electricity costs me 8 cents kw off peak, 12 cents peak. Heat pump will basically pay for itself in 2-3 summers - and that is comparing having the pool constantly at 85 with heat pump vs heating only on various weekends with propane. Just to run a test in regards to heating speed, I heated my hot tub using gas (85 degree starting point, 80 degree air temp) and it took about 30 minutes. Same test using my heat pump took about 60 minutes. Running both simultaneously (just in case you are in a hurry) took about 20 minutes.
My biggest reason for getting the heat pump was to have nice warm water mid May - mid September. I am sure the heat pump will use a LOT more electricity any time before that due to heat loss and HPs not being nearly efficient in colder temps. I am very happy with the outcome thus far. I have barely used the gas heater since the HP was installed. Honestly, when the gas heater fails, I might not even replace it since the heat pump can still get the hot tub going in a reasonable amount of time. But, it could still come in handy when we want to get the hot tub going quickly or if we want to have a party in early May or late September.
Anyways, I thought I would share my experience since there are so many people like myself that debate gas vs HP and not many people that have both that can do a direct comparison. Of course, if you have natural gas available, the decision gets a little muddier as it is significantly cheaper than propane.