Re: polyester vs. polyurethane
I've found more info on these finishes at furniture and cabinet making web sites. Polyester seems to requires mixing three components together and then spraying it on quickly as it has a very short ( 30 to 60 minute) pot life. It is a very high solids content, 80%, application so it can build in thickness quickly. The way some sights describe it you would think you use it to encase things to make them indestructible. Some of the fine furniture web sties I've found discuss the impossiblity of doing a finish repair on a polyester finish, they like nitro too.
Polyurethane seems to be getting more complicated when using it in industrial applications. Seems the stuff in spray cans has formadehyde, so they use different formulations for industrial finishing. It seems you have to add something to the urethane liquid to start the polmerization reaction and the mixing needs to be precise. This too can build a thick, tough finish.
So I think it comes down to which is easier to work with at the guitar factory, number of guitars you coat per day and the skill of your workforce.