First stretchy is relative.

Dacron and mylar both stretch. Mylar does not stretch as much.

Dacron has the same strength and stretch in all directions mylar does not. That is why layout on mylar sails is important. The material has to be turned so that the load is in a direction where it is strong.

Forgiving on cat sails. If you don't have everything set just right on a Dacron sail, the load will stretch it out a little. With a Mylar it will wrinkle and look like you have too much of something and not enough something else. Also if, the Sailmaker slipped up a little or the fabric gives in a local area Dacron will stretch. If a small section of a mylar main yields, it can look very ugly.

All this being said, the best sail I have ever seen is an Ullman pentex 18 sq, a scaled version of the new rig Tornado main