In general:
Polyurethane is more protective and durable, but may be susceptible to deterioration from UV.....excellent indoors, problematic outdoors.
Spar varnish is more flexible, making it more forgiving of seasonal wood movement....designed for boats, and other outdoor applications.
The UV absorbers added to some polyurethanes only last 2 to 4 years. Unless you intend to refinish frequently, my choice would be a spar varnish.
Avoid water based finishes, regardless of the labeling, they're not really varnishes, and they're less durable.