Gregg,
Let me qualify the poly point to say from a tone perspective the more resonant the wood the more of an effect , and the thinner the coat the less of an effect . On a solid body instrument a thin coat of poly shouldn't have a dramatic effect on tone. There is no doubt in my mind it is more durable. Nitro is actually outlawed in a number of states .
When I had the back of my 79 Les Paul Custom re-finished after 14 years of heavy use , I had to have the Luthier do it in his home shop as Cal state law forbid him from doing it in the store ( Gryphon Guitars a great store by the way) .
All of my 11 instruments have Nitro including the 3 I've built. So I'm a bit biased. It is "old school" but I love the way it ages.