Polyurethanes dry fast and cure hard. They are less flexible, but on epoxied surfaces this isn't very important, as the epoxy locks the wood down (assumed encapsulation). The ability to apply three or four coats a day is appealing too. The good stuff will cost just as much as good varnish. I'm paying $130 a gallon for poly. Be sure you get what you need, as the polys are much clearer and have less color then the real varnishes.
Varnish isn't as durable, nor as glossy as polyurethane. It's easy (relatively) to repair and will flex with the wood. The good varnishes will add a lot of color to the wood and they can take what seems like a life time to fully cure.
Polyurethane will out preform even the best varnish, in gloss retention, UV protection, etc., some times by 2:1, but mostly by about 50% as much better. In the sunny Florida climate, good varnish will last a year, before you need to make repairs, polyurethane about 18 months. They both will still look good at these time intervals, but damage will begin to show and if you don't "catch it" when you can, you're screwed and the whole finish and likely some wood will need to be removed, restored and refinished.