The used boat I bought had a dacron jib and a mylar main sail. When the class began allowing mylar jibs, I was one of the first to get one - I like it very much better than the dacron version. It seems to keep it's shape better in the puffs. I may have had a too-well-used dacron jib and a more modern shape was used for the mylar one, but I have been very happy with the mylar.

I should note that I own a Mystere 4.3, on which the jib is very small (an 85 or 90% jib that doesn't overlap the main sail).

Best of luck - those Isotopes look like fun boats.