A mylar (composite) sail is much more durable and stretch resistant. It will hold it's design shape longer. However, it is more brittle and will respond a little less favorably to flogging or abrasion. If you have an overlapping jib (one that sweeps the mast and rigging as you tack) it is better to go with a dacron jib. Unless money is a problem, I would go with a mylar main and a dacron jib. I've never heard of a difference between the two downwind - they should be comparable in this respect.