Actually, a solar cover is a bit of a misnomer. It's main temperature benefit is insulating the water, especially at night. Solar blanket would frankly be a better term. The pool warms up by day and the cover slows the heat loss at night.
Solar panels on the other hand actively heat the pool. The name of the game with those is square footage. The more there is the more solar energy they are collecting and the warmer the water. I don't know what you run schedule for your pool is but with solar panels you will need to have it running during the day and off at night to see the maximum benefit.
You probably will want to either get the solar cover alone or get both, since you can lose a lot of the heat you generated during the day overnight without a cover.
