If you have the $$ go for the larger size. Run to the bottom. If you have a wooden base. You can attach to the base with a long wooden board and deck screws. Easy to remove each year. If you read my previous post - please do - you can see there are many ideas and issues. If the pool cover goes to the ground on each side there is less possibilities for the wind to go under and cause it to fly off. Do secure tightly - It is not much fun to see it fly off in the middle of the night and have to go out and resecure because you think it might blow away.
Tape is ok - But will it hold up in the extreme cold and the wind. ?? I would use more than tape.

Thickness - I am sure there are lots of reasons why to pick one vs another - for me it is what ever I can afford - and will you recoup the extra cost in heat savings. Mike

Even better: don't run to the ground. Run to *below the ground*. Treat the ground as a heat sink. If you insulate the ground's connection to outside, heat can't flow from the ground to the outside very quickly at all. A few inches of soil doesn't insulate well, but a few feet certainly does. Even a foot of underground insulation will be a big help. Also, having it embedded in the ground makes it much harder to fly off.

Note that you don't need to have it go straight down in the ground. I'd recommend going down a few inches, then outward. It's easier to install it into the ground that way, and you isolate off a larger chunk of soil as "warm" than you would with vertical insulation.

Of course, underground, you don't need transparent insulation; you just need non-water-permiable, high-R-value insulation. Of course, you should have that on your north and (optionally) side walls anyways, not a pool cover. :)

Why tape? Is this not going to be permanent? I caulk everything on. :)