That depends on what you want to do with the custom. I assume you mean by polymer clay you mean clay you have to bake in the oven (like Sculpey) and by epoxy clay you mean the stuff that hardens within a certain time limit (like Epoxy Sculpt). Epoxy is good for customs that you don't want to risk putting in the oven and/or that you can sculpt the design in clay you want before the clay hardens on you--unless you work fast, that means simpler sculpts. Epoxy clay has a limited range of colors (from what I have found) and will probably need to be painted. In my admittedly limited experience, I think epoxy is also a bit sturdier than polymer clay. Polymer clay has to be baked in the oven, so if it's attached to a pony, you need to make sure she and everything on her (if you do any work before adding the clay) can survive the baking process. Polymer clay comes in a huge range of colors, so you may not need to paint it when you are done. You can also work from days to weeks to month on your sculpt with polymer clay without it drying out if you seal it in a container.