Depends how you plan to use it. I have both, one at two seperate properties. Generally, if you're going long stretches without it, and just want to heat up the pool for the weekend or that upcoming party, gas would probably be the way to go. It'll bring the water up to temp faster and the unit itself is most likely less expensive. If
you're looking to keep the pool at a constant temperature throughout the season, the heat pump is probably going to be more efficient and less expensive over time. Because of how they operate they're not as effective below 65°ish, but the water is still plenty toasty. My personal experience with mantenence is that the heat pump has needed zero attention in the last 7 years, heat pump has needed attention twice in that same time frame ( bad igniter and control board issue). But thats just my personal experience, ymmv.