Great question. I have heard that Howard Hill preferred Marine Varnish. I personally like the matt finish, and the advantage of using marine varnish is many. It can be brushed, sanded, coats added, and if needed repaired. Most finishes will show scratches, and for me, I like to be able to fix a problem without having to refinish the whole bow.
Glossy finishes are pretty and look nice when new, but have you noticed how some hard glossy finishes tend to develop surface cracks with time? I have several bows like this that need to be refinished, and a couple that have yellowed with age. I want a finish that will avoid these kind of problems. Marine varnish and Tru Oil look like winners. I have also heard that Daley's Profin would be a good one to try.