I've only had the propane heater since last April, so I can only give you rough numbers based on one season. And, my climate is a bit different than yours as well, so YMMV.
My heat pump works really well during our swim season, which is basically late May thru the end of August (if you dont have a heater). I keep the water around 84 so in mid may to mid June and late august to early september, the HP runs quite a bit to maintain that temp, maybe 5-6 hours a day. And, sometimes i cant get 84 degree water temps either, even running it for 12 hours. But, our low temps even at that time of year can get down into the mid 50's and take awhile to recover. A heat pump needs consistently warm air to work well. Now, if you like your water a little cooler, it might work better. In general, what I've found is that in real world situations, a heat pump works well with air temps above 65 degrees, and better at 70+. The longer it takes to get to 65-70 in the morning, the longer it will run, and the longer it takes to heat the water. Cool, rainy days and you might as well turn it off. As I said, I like it, but at least up here in Massachusetts, it doesnt extend my swim season at all. For you, it would probably extend by a couple weeks on either end, give or take, and in the summer, it would work realy well. Again, it depends on how warm you want the water, and how many hours during the day the air temp is above 65.
I got my propane heater last April since i wasnt satisfied at all with the HP in late April, May, and late August-mid October. We get some really great swim weather then, but not enough daytime heating for long enough for a heat pump to work well. For example, we might hit 80 degrees in early May, but the low might be 52 and take until 2 pm to hit 80. So a heat pump is pretty useless. I used my propane heater in May of last year pretty exclusively, as well as late august and all of September. I dont keep it continuously warm though like i use the heat pump. I used it on warm weekends, or 1-2 times during the week depending on the weather and if my kids wanted to swim after school. Even during the summer I used it a few times when I got impatient waiting for the heat pump. In general, I'd put around 8-10 degrees in the pool each time i ran it, sometimes maybe 4-5 degrees in the summer. So in one season, I used ~200 gallons of gas. I pay $2.75 a gallon so about $550 worth of propane last season. It's not cheap. If I had to use propane all season with no heat pump, it would get expensive. Thats why i got the heat pump to begin with. I dont have NG on my street (cheap builders!) so i'm forced to use propane. If I had NG, i wouldnt have the heat pump. For my NG rates and electric rates, NG would cost a touch more than the HP, but not tremendously so. For me, a HP and the propane heater are a nice combo. But, i also spent over $7000 for both heaters, electrical hookup, and the propane install.
