We also considered installing both a heat pump and a gas heater to minimize monthly running costs. In Virginia, the pool really needs to be closed over the winter because the temps range between 18 and 40 degrees for a few months (though this year we had a very warm winter). But, the goal was to be able to extend the season so we could swim April through the end of October, including outdoor temps averaging 55+. Also, I wanted to be able to maintain a fairly constant temp every day during swim season.
We ended up opting for a heat pump only, reasoning we could add a gas heater later if it turned out to be necessary. I am so glad we did, as the heat pump alone so far has been perfect. We opened the pool on Wednesday with water temp at 61 and set the heater to 80. Within 24 hours it was 72 and within 48 hours it reached 80. This was with day time temp of 55, very over cast, and nighttime temps in the low 40s. The water temp rises about 1 degree every 45-60 minutes during the day when temps were in the 50s (again, without sun). Our pool is 30,000 gallons (20x40, from 4 to 6 feet deep) and does have an autocover. We have been closing the cover at night to reduce heat loss.
We do not have a spa and during the summer pool temps can reach mid 80s or 90s from the sun /heat alone, so I don't think the quick heat ability of a gas heater ultimately would be important for us. Gas can be prohibitively expensive in our area and I hated the idea that someone could accidentally turn the heat on high and run up a massive bill. Even if we ran our heat pump 24/7 for a month the bill would be hundreds, not thousands, of dollars. In addition, our heat pump has a chiller feature that can be used to cool the pool in the hottest months.
When I looked into which type of heater to buy, many said gas is the only way to go. But a lot of the folks I asked also had no firsthand experience using a heat pump and consistently said it could not be used to extend the season because they perform less efficiently in cooler temps. As I said, ours is doing great with day temps in the 50s and nights in the 40s in completely overcast and rainy conditions -- but I think the autocover helps a lot as well. We also opted for a large unit (the RayPac 8350 / 133k btu), which is one size larger than the minimum needed for a pool the size of ours.
One downside to the heat pump is that it cannot raise the temp as fast as a gas heater. I also think you may be able to reach higher temps with a gas heater. We only set ours to 80. I am wimpy about cold water but I think 80 or above is fine for swimming, fwiw.