Solar can be used in conjunction with another heat source. You run the solar as much as needed, and supplement with the other source as needed. Gas would probably be the best choice as a supplemental heat source if you are installing new, as it usually heats faster and has a lower initial cost. Since the solar runs at virtually no cost, you run it almost all the time.
My system consists of 6 8'X4' Aquasol panels on the roof of my poolhouse. The poolhouse is about 6ft from the pool, which makes it ideal for both an equipment shed, a solar panel holder, and a place to get out of the sun while watching the kids swim. I chose the brand because they had 4'X8' panels that had reasonable performance, and the panel size was driven by the size of the poolhouse.
I don't have an automation system. I run with a mechanical timer that turns on about the time the sun hits the roof in the morning, and shuts off about the time the sun drops behind my barn in the evening. I keep an eye on the forecast and pool temperature and set the valve for solar accordingly for the next day. I can usually get by without my solar blanket when the overnight lows are in the 70's. It's nice just to not need the blanket. If I automate, it will probably be to keep from overheating the pool as I did a few times last year. My personal "ideal" temperature is about 84. I had a few days last year that I picked up around 10 degrees in a day, so I went from too cool to too warm in one day. I use the blanket when I'm trying to increase the pool temperature, or if the forecast calls for cooler than normal temperatures or wind. No big deal, just a little bit of planning.
The plumbing is simple, as is roof installation other than the scuffed hands and sore knees from the shingles. If it fits your plans and space, making your solar rack 8 or 10ft tall gives you a nice shaded patio with only a little bit of extra work and money. The panels can be mounted close to flat. A little tilt is handy to make them drain when you aren't using them or when winterizing the pool. You could go as simple as an area of pea gravel with the panels laid out flat on anchored sheets of outdoor plywood to see if you like the way they heat before investing more money in it.
IMO the downsides to solar are the space they take up, which is mitigated with an elevated installation, and the fact that they can't heat well on cloudy or windy days. I'm thrilled with mine.
