Difference between Epoxy and Polyester Resin
There is great debate on usage of these two materials, here is my simplified opinion.

Polyester Resin
Epoxy Resin:
Good Properties:
Bad Properties:
Good Properties:
Bad Properties:
1 - It is cheap, currently about $20 a gallon at Home Depot. 1 - It does not adhere to wood very well. If you pick at the edges of fiberglass coated with polyester resin, with some force you can pull the fiberglass completely free from the wood. 1 - It is less expensive than it used to be, can be purchased for $35 per gallon. 1 - Not usually available locally and has to be mail ordered, but that really only adds about $10.
2 - It bonds to itself very well when applied in one setting. 2 - It has very poor secondary bonding, even to itself. If you try to apply additional coatings of polyester resin to fully cured previous layer of resin, the new layer could completely seperate from the original one. 2 - Has excellent bonding properties, even bonds very well to polyester resin. 2 - Starts to go soft at 130 degrees
3 - If used as a coating over a plywood boat either by itself or with fiberglass, it seems to provide a good durable surface. 3 - It stinks very bad while curing, wear gloves / protective clothing / respirator and work outdoors ! 3 - Adjustable cure time depending on how you mix it. 3 - Not UV resistant, needs a coating of paint to protect from sun.
4 - It is UV resistant by itself. 4 - Has a very fast curing time, you only have 15 minutes to work with a batch. 4 - Lot less stinky than polyester resin, but still can cause allergic reactions so wear your respirator and protective clothing & gloves! 4 - Needs a lot of heat to cure, building in the winter time often requires that a lamp be placed next to the area you just worked in, the lamp provides heat for the epoxy to cure.
5 - Cures quickly, so you can apply multiple coats and layers, and sand in the same day.     5 - Has a blush (oily surface) after curing that needs to be cleaned off before other layers can be applied. NOTE: many new types of epoxy do not create this blush.
6 - Doesn't need as much heat to cure as epoxy, will cure in colder temperatures so you can build in the winter.      

Epoxy is cheap enough now and has enough advantages that it would be hard to
justify using polyester resin in a plywood based boat. But, if you really
want to give polyester resin a try, go ahead. You can always fix the
problems with epoxy over top of the polyester resin.