Maybe I don't know what I'm talking about, but I think your question would apply primarily to the mainsail, because regardless of what the main is made of, most people want dacron for their jib. I may be going out on a limb here, but as a crew, I need to be able to "read" the jib, and it needs to be a "soft" material.
And what I have heard about mylar mains is that they are the best IF they are cut right in the first place, but if they are not, it is a little harder to change the shape and the draft by altering battens, than with a dacron sail.