I'd personally use polyurethane because it is harder...but spar urethane is a great choice as well. You can buy "wipe on" polyurethane, but that is more expensive. Like I said, I mix it 50/50 with naphtha (you can you other solvents like mineral spirits, but naphtha dries quicker). Best recommendation I've seen for applying it is "do it like the kid a Denny's wiping the table". The more you go over it, the worst it will look. Put on one coat with a lint free cloth. Let it dry, do another coat. After about 3 coats I lightly "sand" it with the finest steel wool (or 400 grit sandpaper) just to get dust particles out. Wipe the dust off with a dry cloth or a cloth with just a tiny bit of thinner on it - just to give the dust something to stick to. 2-3 more coats, sand & clean again, then a final coat. For that last coat, the original jar of my mix is almost empty so I pour in a little more polyurethane but not more naphtha so that final coat is probably more like 75/25. Sounds like a lot to do, but it goes very quick (recoat in 30-60 minutes) and doesn't take much skill - as long as you do thin coats and don't keep wiping over it. With regular polyurethane, you brush it on and it is easy to get bubbles and brush marks...it is hard to get bubbles with thinned wipe on.
Spar urethane is supposed to be easier to apply. You can use the same method I describe...I wouldn't thin it quite as much and that probably means a coat or two less. Spar urethane is softer than polyurethane, but better water and heat resistance (not that polyurethane is easily damaged by water and heat...just that spar urethane holds up better to those).