Polymer clay works on a completely different principle from epoxy putty. Nothing is mixed and nothing starts curing, so the material stays soft for as long as you want to work it — days, if the sculpt needs it — and hardens only when baked at a low temperature. That makes it the natural choice for large or slow sculpts, and the wrong choice for anything attached to a styrene kit, which will not survive an oven — a room-temperature epoxy is the material for that job.
