: I could choose
: between 16 Oz. PVC (made of _polyester_ core coated both sides
: with vinyl) or 13 Oz. PVC made of _fiberglass_ coated both sides
: with vinyl.
: The 13Oz seams tough enought but less flexible. The 16 Oz looks
: more "gummy".
: Which one is the right one?

My experience is with Vinyl coated polyester. I have no experience with vinyl coated fiberglass.
Vinyl "laminated" polyester is also avalable, but it tends to be less durable than Vinyl "coated" Polyester.

The coated PVC / Vinyl I use is generally 18oz, though 16oz should work ok also. I've used 10oz but it easily developed pinholes.
The problem was likely the vinyl coating, not the weight. It's difficlt to make an absolute statement here as there can be a huge
difference in the quality and flexibility of vinyl coatings being used, and even in the woven polyester core material.

I used Coverlight 18oz PVC for years and it was very durable and fairly flexible ( for PVC). It went out of production and was replaced
by Coverene 18oz. It was stiffer and less abrasion resistant. I switched to RSI 18oz ( Rockport Supply in the US), and it is pretty good,
but still not quite up to the quality of Coverlight.

Coated Polyester tends to be pretty stiff, so the vinyl coated fiberglass being even more stiff as you say, may be a problem.

I have never seen gummy PVC / Vinyl ? If you ever see vinyl coated nylon stay away from it as it will shrink rendering your folding kayak skin useless.

Tom