Of course, you have to figure a few other things in:
1) Initial cost. HPs cost quite a bit more. But they are easier to install, needing only electric, not electric and gas. How long will it take for the cost to be amortized by the fuel savings?
2) Which ones are the most reliable. Can't help you there.
3) You can't really run them intermittently as they take far longer to warm up a pool than a gas heater. I think the max BTU is about 125K, with a 100K being typical, while gas heaters go up to 400K BTUs.

This isn't research but things to watch and check as you do your research. Hope it all helps.