You're probably going to have to get 2 covers-one for each "leg". If you want to use a reel you will probably have to get 2 reels also. You could make it one big cover by using solar blanket tape, but it would probably be too heavy for a reel as one piece (you'd have to fold one of the "legs" over to create a rectangle in order to roll it up on the reel).

As far as which "type" of cover is better...I don't know. I've got a blue 12 mil and it keeps my pool a good 15 degrees warmer while the cover is on (at least on the top few inches of water...once we get it and stir it up the water temp falls about 8 degrees, but it's still warmer that it would be without a cover). No additional heater on our pool.