Check out the discussions on the Wooden Boat BB.

Some folks there have strong preferences.

If I remember correctly, the main points are that varnishes with urethane fail throughout the whole thickness and crack. To repair it, all of the finish must come off. Varnishes w/o fail at the surface and gradually wear off. To refinish, they just need sanding and recoat.

Also, with the Interlux varnish I used, the urethane took at least 2 days to dry before it could be sanded, the varnish w/o dried in a day. The urethane was MUCH harder to sand.

I am now not using a urethane varnish. the 1st boat has some of each and I'll watch it to see if I can tell any performance difference.

Dan