I'm on the same page as BESSLB regarding thickness. Over the years I have sewn my own windows as well as several complete tops made by contractors. In my experience the thicker material outlasted the thin by a very wide margin. I agree that the 20ml is saran wrap. My last boat I had the front windows made out of 40ml and the side and back windows out of 30ml, at 7 years of northern climate use they were tired looking and beginning to rot and at 9 years they were toast. The boat before this I went through 2 sets of 20mil windows in 6 years.
My present boat has Isenglass which after 12 years of Florida sun is still in reasonable condition, no yellowing or rot, just heavily scratched. The glass is actually in better shape than the vinylized canvas it is sewn to.
Sue, a couple of things I look for in a well made top is:
-extra width valances / flaps covering the zippers
-waterproof tape sewn into horizontal seams and zippers
- nice straight crisp lines and tight when all is buttoned up versus having to bulge it out all over the place with the stainless frame just to get it all tight.
- The raw edges of all windows have binding sewn on and not just simply sewing the window to a zipper or the fabric. ie: the raw window ege should be sandwiched in between material.
-backings behind all fastners
I'm sure the experts will have other tips. Good luck!