Depends on what you are looking for. I like varnish, but I like oil
too. Like I said it is very rot resistant so if you are not particular about how pretty it looks you could leave it bare.
Spar varnish would be fine, I am sure it will look real nice. Oil
or wax would be a lot less work.
Maybe you could try different finishes on a small piece and see which one you like the best? The color change happens faster in the sun light so you can put the finishes on and them put them in the window for a couple days or a week depending on how much sun you get.