For the last few weeks I've been using a Palm Pilot with Wi-Fi, instead of my laptop. Just discovered there is a limit to the text size with the browser I'm using. Gotta do this posting in two parts.

Anyhow, back to the fiberglass:

Just about any glass recipe commonly used for fiberglass fabric will give a cloth which is suitable for our use. The stuff they use for fiberglass insulation is not so good. There are dozens of mills around the world which weave glass fibers into cloth. The stuff is a commodity which has some variations, but like cotton cloth, one brand is pretty much interchangeable with another.

The general guiding principal is that the more glass you have, the stronger it will be. Usually you want a mix which is about half glass and half epoxy resin, by weight, foran optimal weight to strength. If you use a stronger base, such as the stronger glass used in S-cloth, you can use a lighter piece of cloth, and less resin,, than if you used E-glass of the same strength. So, some people use 4-ounce S-glass in places which call for 6-ounce E-glass. Strengthwise that seems to work just fine. Although for our purposes, most of the time you can replace 6-ounce E-glass with 4-ounce E-glass and have a perfectly fine boat, too! But the thinner sheathing of cloth (either E or S) and resin is a little more prone to dents and dings.
A superior glass cloth will be tested, go through some quality-controll steps in manufacturing, and you should be able to get hold of these specs before you buy. Even the best materials can be bought on sale. Some materials made to meet certain aviation or military specs may have production overruns, for example.

The least expensive brand sold by reputable marine suppliers is going to be good enough for your building project. In most cases this is the stuff the designer used for their prototype, and if it worked OK, then that's what they call for in their boat designs. If you can buy something better for lower cost, then go for it.
Hope this helps
PGJ