Go with any of the above. The main purposes are to provide a glosyy finish by filling in the swirlmarks on the sanded epoxy, and to provide a UV absorbing layer to protet the epoxy from an early death. All the outdoor rated varnishes I've seen have UV absorbing materials in their mix. These would be polyurethane varnishes as well as resin based varnishes. I believe the minwax Helmsman product calls itself a urethane varnish. That would differentiate it from a polyurethane varnish. Unless you are taking a chemistry test, you could probably lump it in with the polyurethanes. For our purposes they work about the same.

PGJ

Usahardware.com has the following blurb about the Minwax product.

(http://www.usahardware.com/inet/shop/item/10900/icn/20-942748/minwax/63200.htm)

"Helmsman" Spar Urethane Varnish
Covers approx 100 sq. ft per qt.
Tougher than polyurethane varnish
For exterior doors, bar tops, window sills, etc.
Urethane fortified alkyd resin, formulated to dry hard, but not brittle
Interior or exterior use