I'd PREFER to have both.......but I just bought my first solar cover earlier this week. Common consensus here is that there is no relative difference between performance or longevity between 5 to 16 mil. blue or clear or the mixture of the two. The cheapest 8 mil you can find is my recommendation. The covers mostly shine in maintaining temps you have rather than adding to them. I haven't had any warmer ambient air temps than the pool temp since I put mine on, at least long enough to matter.....but I'm waiting to see!
The first step is always to add a solar cover, 8 mil, thin and cheap. It may do all you need. It will add 2 or 3 degrees to the pool most days by reducing the overnight heat loss. Over time it will tend to keep your pool at the high temp rather than the midpoint between low and high. Solar panels will add heat to the pool. Our pool has been 88-89 for the last couple of weeks with a good high 80 and avg low 60. This mostly with solar panels and a solar cover and a little boost from the heat pump during cold fronts or rain.
I prefer both as stated earlier but the costs for solar panels are much more than any cover. I'm looking to keep a consistently high temp of 90 year round in florida. As stated the solar panels add heat, the cover retains it especially overnight and during the cooler days. The cover will also create a cumulative effect (the longer it's on the more heat will be saved) and it is also great to prevent evaporation. I had no interest in liquid alternative based on the reviews and I really don't want to pour anything like that into my pool.