There is a great deal of confusion about epoxy and "fiberglass" - a number of vendors and even some of the rep's selling manufactured boards would describe Epoxy boards as different from "fiberglass" boards. If you ever READ that - they are totally full of BS and go somewhere else to buy your board.
Epoxy is a resin, polyester is also a resin and these are the two main resins used in laminating fiberglass (or other fabric like Carbon Fiber) to a surfboard/wakesurfer. The comparison then is an Epoxy board or a Polyester board (not fiberglass). All current epoxy surfboards/wakesurfers contain fiberglass (or some other fabric). Despite all the hype from manufacturers, epoxy is a medium soft plastic and needs fiberglass or some other fabric in the matrix to provide any sort of stiffness and impact resistance.
In ding repair kits there are polyester kits and epoxy kits as noted above. Polyester contains styrene and will DISSOLVE an EPS foam core, which is the most common core of epoxy boards. Epoxy will work on anything without causing damage. That said, if you aren't certain what your board is made from, choose epoxy to be safe, it will stick just fine to a polyester finish.
In most epoxy ding repair kits, they are selling a fairly good quality 5 min epoxy. As Laker1234 points out, the cheap 5 min stuff from Walmart or your local hardware store works just fine, but will tend to have an amber color typical of low cost epoxy hardeners. If that isn't a problem for you, you can save $10 off the ding repair kits. Everyone should carry a small package of 5 minute epoxy in their boat for the enevitable ding.
Principally what you are paying for in the kits is the two pieces of sandpaper, two popsicle sticks
, two pieces of shiny mylar, two plastic dixie cups to mix in and a few square inches of fiberglass. Maybe $2 worth of product, above the epoxy and the convience of having everything avaqilable.