Go with Z-poxy it is foolproof. Polyester is great for fuselage, cowls, etc. If you want to sheet wings, you need to be really careful that you have a good seal on the sheeting otherwise the resin will destroy the foam. Polyester is very temprature sensitive, if it is too warm, it cures quickly, if it is too cold, it remains tacky. If you put it on too thin, it stays tacky. Pot life is really short when you mix fairly large quantities with a fair amount of hardner.

By the way, Z-poxy sands a lot easier than polyester.

Good luck

Zane