also picking the right resin involves knowing what you're applying it to, and then what you're going to apply over the top of it. using an epoxy and then using a poly "flow coat" wont stick.
Better reaction time than me, just as I was about to post that.
Epoxy is very tough and flexible, but it is not compatible with polyester resin. Some epoxy is so strong that you will get a very good mechanical bond to the original glass, but a chemical bond is not possible between polyester and epoxy.
If you put down a lamination using vinylester resin first, further laminations with epoxy can follow. This will promote a chemical bond as well as a mechanical bond between the polyester and vinylester laminations and between the epoxy and the vinylester laminations. Basically, the vinylester resin lamination acts like a primer between the epoxy and the polyester resin systems. The way it was explained to me by the chemists who formulate the resins is, one side of the vinylester resin molecule looks like a polyester resin molecule and the other side looks like an epoxy resin molecule, allowing a molecular bond to occur.
Word of caution: there are so many epoxies available now, make sure that the one you choose is compatible with your intended repair. It is possible the whole repair could just release from the original glass.
I think I would stick with the polyester resin system as that is most likely what your original glass was laminated with. To promote a complete cure of any polyester resin or polyester gel coat you apply, you can add styrene wax to your resin or gel coat. Here is a quote from
http://www.fibreglast.com/product/Styrene_Wax_71/Polyester_ResinsGet a Complete Cure
# 71 Styrene Wax is a 5% solution of paraffin dissolved in styrene that may be added to non waxed polyester resins and gel coats to prevent the prolonged tackiness associated with thin sections of polyesters. If the resin contains paraffins, the wax will rise to the surface and protect the curing resin from the inhibiting action of the air. However, if the resin is allowed to fully cure, the paraffins on the surface will prevent adhesion of further coats of resin and must be sanded off prior to additional applications. We recommend adding #71 at 5% for thin coatings. Use with #1040 Hi-Gloss Additive when performing repairs below the water line.