Should I get a solar cover or a solid thermal cover for preventing overnight heat loss in my pool?

Get a solar cover (the bubble wrap kind). It’s the best for preventing heat loss overnight because it traps heat in the water and cuts down evaporation, which is the biggest source of heat loss at night. A solid thermal cover is heavier, more expensive, and doesn’t let sunlight heat the water during the day. Thicker bubbles (12 mil or so) work better than thin ones.