My experience is that though the urethane is more abrasion resistant, it doesn't help much. Sand & rocks are pretty tough too. Practically speaking, there isn't much difference in abrasion resistance and the urethane based stuff is more brittle and prone to crazing over time.

I'd stick with the phenolic resin based stuff such as Zspar's Captain's Varnish. Unless you leave the boat laying around in the sun ALL the time, you don't need the real heavy UV resistant stuff which also has it's downsides. Revarnishing every couple years, before it starts to break down is the norm and with regular spar varnish is pretty easy. If you use the urethane varnish, refinishing is more difficult (Hard to sand, more resistant to abrasion :>)

Everyone is always trying to come up with new mousetrap, but most of what we know about good varnish finishes on boats was figured out a hundred years ago. Stick with the old fashion stuff, you won't regret it. Craig