Hi Ross, I overbuilt my first s&g with 5.2 oz S glass and way too much epoxy. The local TAP Plastics store had misc. cloths so I experimented and used up $200 on pricier cloth. A few years later after building a few more kits I made up about two dozen 4mm ply test panels with various permutations of 4 oz E,S, 6oz E., 4oz polyester, 4oz kevlar. and basically bent,hit and punctured them.
S glass fractures and propogates fractures differently than regular E-glass. For example two panels: one with 4oz E on both sides, one with 4oz S on both sides. Bending the panels they both bent about the same distance but the E glass would fracture with long cracks along the threads with permanent damage to the fibers and wood. The S glass panel would bend a little farther with fractures in the surface traveling a shorter distance. The surprising part though was how far the plywood would bend and return without total failure. I think this is called a high propogation energy. Impacts on S-glass panels resulted in a smaller pattern of fractures from the point of impact and the fractures were less likely to follow straight along the lines of the threads as with the E-glass panel. Of course using 6oz E glass the panel was much stiffer and stronger at 1/4 the cost. The characteristic that I find interesting is the slight elacticity the fabric has compared to E glass.
I built a friends Coho entirely with 4oz S-glass with double layers on the bottom panels and made an appreciably lighter boat. But still I also was very careful with the amount of epoxy used. One could be sloppy with an expensive material and make something that weighs the same as with heavier cheaper cloth and quite possibly the cheaper one could be stronger just from a neater and more careful construction technique.
I think the abrasion resistance of S glass is higher than E.
As far as handling and wetting out, I'm under the impression that a similar weave of S is slightly stiffer. But I don't know if that is because of the cloth or the sizing (coating on the glass that improves bonding with resin)
I think Paul Jacobson or JR Sweet said that there is a shelf life to the coating on S-glass that should be taken into account as a lot of S-glass characteristics might be degraded with old cloth, I'm not absolutely sure on this one, maybe someone else can chime in.
If money is an issue it's not worth it. You can do a lot with putting glass where it's needed neatly and not where it's not needed. But if you want to make something light then S glass has a place.
The cloth I used was a regular weave and it wetted out fine, but the threads seemed a little more visible than E glass in the cured epoxy.
The thing to keep in mind with glassing panels is that panel rigidity will be addressed better with a structural element like a rib or ridge more than simply a fancier fabric like S glass,kelar, or carbon. Then again using those materials with that element takes things a little farther. Like a couple thicker strips rounded on the inside with a thin layer of cloth over that.
I'm thinking of making my next boat with various layups of 4oz E and S glass. The funny thing about all this fabric fetish is that when the boat is finished it'll still be heavy at the end of a long paddle, immaterial if loaded, or damaged by carelessness. Sure go for it, but don't get the high thread count stuff for glassing.