Chuckc, , ,

They are both basically variations of polymers. One uses an epoxy catalyst while the other uses MEK mixed in at about 2% per volume.
The polyester variety is most common in boat manufacture, and other high production applications because of it's easy use with spray equipment, and the ability to control it's final hardness. Yes, technically you can spray epoxy, but you also run a serious risk of hardening up the spray gun! In the case of polyester however, the MEK is either injected or atomized with the resin outside of the gun. Or the lower budget version of counting out drops of MEK.

As a hardness comparison, I would say that the epoxy would be somewhat harder. If applied too thick, it would fall into a rehlm of being brittle.

Note the spray gun that I'm using in the picture. The plane is the XP-67 Moonbat. The whole thing is done in about 10 minutes using about 2 ounces of resin!
The spray gun is unlike anything thats used in the auto industry.
Spraying polyester requires very special equipment that will set you back about $1,300 to get set up (providing that you already have a suitable compressor).

Cost is probably the predominant reason that most modellers just use epoxy.

In-so-far-as modelling goes, I like using polyester. I can spray a very thin, dusting coat on the fiberglass cloth that requires minimal scraping at all. Most of the time, I never need to scrap it out anyways! Go for lunch, come back and pick up the gun and start spraying again! Clean-up is very easy using Gunwash. You can control the hardness of polyester by using an additive.

If your going to consider polyester, and you have the funds, counting 4-5-6-7 drops of MEK is not the way to go. I've been set up for spraying for along time and would find it hard to look back.