You can get a basic overview here:

http://www.pettitpaint.com/catalog_browse.asp?ictNbr=56

A few years back I had an in-depth conversation with a sales person from Epiphanes.

Basically there's very few actual varnishes made these days. Epiphanes is one of them. A varnish is all natural like the description of home made varnish on the James Town site. Turpentine, Boiled linseed oil, coal tar and Jaπan dryer. Most of what is called varnish now is polyurethane. Polyurethane just means synthetic or plastics. Polyurethanes are harder than regular varnish. They were originally invented for the flooring industry. Regular varnish is actually more flexible. Most exterior varnish is actually polyurethane. Marine means they can charge more. Regular varnish is used a lot on boats over mahogany and has to be able to take the contraction and expansion of wood. We are varnishing over epoxy so this flexibility is really not necessary. Poly is probably the best choice but I really like the Epiphanes varnish which is a true natural varnish. Generally strippers get re-varnished not because of varnish failure but abrasion and scratches. I think the choice in many ways has to do with how the varnish flows and ease of application more than it's useful life for us. They will all look great for a few years. Your question is actually a pretty technical one and you can go by others recommendations and do some research yourself.