In an average of 10mph winds, what makes better sense .The old delaminated but full mylar main or the new pentex which is considerably flatter. On a course that could be a beam reach to a broad reach.Does it really make a difference in performance.Note they are both square tops the pentex is larger up top the mylar is larger on the bottom. M2KL 50years ( open portsmouth handicap ).
I'd go with the pentex on the basis that it has a better chance of surviving, and you never know when you have to go against the wind.
Old delaminated mylar doesn't sound very "long lasting" if the wind picks up or your jibe becomes a bit violent.
Thanx Rolf,I know there appeared to be an obvious answer but I've seen in the past, in some strange circumstances watched old boats with blown out sails, perform much better than expected .
I have an old blown out jib that I would not trade for a new one. it is great in light air, and works ok in the wind. Now my blown main is so brittle that it shattered last weekend when I gently fell on it on my back. (looking for a new one) 
Marc you could epoxy an 80's wind surfer sail smack over that main right up and down the leach and "BAM" good as new.
Actually I was thinking of going to my local Hobby Lobby and buying a roll of acetate, and just pasting it on with some epoxy. 