12 mil is heavier. Retains heat a little bit better. You have to determine what your needs are in a cover and then go from there. Darker more opaque ones help cut back on FC loss to sunlight. Clear ones allow more light to get thru which may heat the water faster than a darker cover. The main thing they do however, is retain heat at night that is gained during the day. I have had several kind, they don't last more than 2-3 seasons regardless of the thickness, etc. so I now don't spend a fortune. I like my blue/black combo.
Can you put your pool specs/equipment in your signature... go to User Control Panel, then Profile, then Edit Signature.