Winter pool cover questions

by Raschel
(Blaine, MN USA)

I was wondering what is better to go with on picking a pool cover because I don't know the difference between the Mil or gauge so I was wondering which would be better to go with the gauge or Mil? I was also wondering what size air pillow to get a 4ftx8ft or 4ftx15ft. I have a 24ft round pool?
Thank you,
Raschel