That fellow/member that works/worked for Interlux wrote a couple of varnishing articles a while back (they were in the Wooden Canoe), and in them he related the following (IIRC);
"Traditional varnish fails/deterates at/from the outside layer, and when it needs refinishing, it just needs sanding and a new coat,
Urethane finish fails by cracking through the whole layer, and needa to be totally removed when it needs to be refinished."
Also, in my limited experience, the urethane is much harder, and sanding between coats takes much longer. I used it on my 1st stripper and traditional on the 2ed, I probably saved a month of time going to the traditional finish.
Dan