You should go for the 18. Yes they are bloody hard to get on and off Yes they work, smooths everything out.
I bought these but it didn't have the strap to attach to the bra, it must be new because I didn't see this version at the time. If I had the cash now I would go this route as mine did tend to roll over at the top when I bent down. Not sure how much of that was due to me being over optimistic on the sizing though and getting smaller than I should have