If you have the space, do solar AND a heat pump/gas heater. With the solar heat, and a solar blanket, you should be able to go all summer and a little beyond--in both spring and fall, with zero operating costs. Solar DOES work, but only when the sun shines. The blanket (I prefer clear thicker ones) acts like a greenhouse.

Personally, if you have access to natural gas, rather than propane, that's a far cheaper choice. If you are talking a grill, the cost of plumbing NG doesn't pay in savings over propane. But in a 400K BTU heater, the cost savings are substantial.

Yet, despite that, if you are talking temp maintenance, you are best to go with a heat pump--it will still be cheaper than the NG, much less the propane. Combine it with solar and a blanket and you'll have a nice, extended season.

I just have a personal bias against gas heating--it seems there are SO many ways to recover heat cheaper and more environmentally friendly. I live in N-Central NJ and I have NO trouble keeping my pool between 85 and 95 solely with solar and a solar blanket.