i'd go for the 12 versus 8 or 15..why? because the 12 is relatively lightweight to manage, yet that much more durable than the 8 which feels to me like a grade above regular bubble wrap i really like the solar cover i picked up. i believe it was 100 bucks for an 18 x 30, easy to cut to size..and it's been doing very well at keeping the temp up about 5 degrees over without the cover which my family tells me is a huge difference (80 is cold to them..85 nice and warm..wusses)