I would agree that you should stay away from the mylar composites. I purchased a mylar jib from hobie for my SX a couple years ago and it currently has less than 25 voyages on it. The jib is not holding up very well since it is showing early signs of delamination. It is very light which would help in racing but for the high price you pay it is not worth it for the recreational sailor. The new main I bought this year from Whirlwind is made from the material that the hobie 20 mains are made from. I am happy with it so far. I belive they can cut you a dacron jib in the SX dimensions if you want utlimate durablility.
82 Hobie 18 with SX rig and seats