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Well Kevin, I would suggest you do a search on the topic here. Preferred is a subjective term, but if you consider the vastly greater number of Varnish finishes over Polyurethane, I guess that you could conclude Varnish is "preferred". A complete explanation of the virtues of Varnish would be long and exhaustive. Polyurethane is actually a synthetic resin and used in the production of Varnish, but the end result is somewhat different.

I would also suggest some reading-BRIGHTWORK, The Art of Finishing Wood by Rebecca J. Wittman. It is an excellent book, easy to read, and very informative. I'll lend you my copy if you like.