I've used gallons of both and while urethane is supposed to be harder, I don't see the difference so I'd use either regular spar or a poly urethane. I'm about 12 seasons on my saloon table which has seen much use and abuse and the spar varnish is fine with no signs of unprotected wood. It like the sole(also just spar varnish) as lost it's gloss and is more of a satin. Below, varnish last decades and decades.

I'm curious about the Kiwi deck myself.