I have played Nitro, Shellac, Poly and Tru-Oil.
The tonal difference I've heard related to the finish, if any, is due to the thickness of the finish, not what kind of finish it is. Also important for me is the durability of different finishes and how these relate to the purpose of the guitar.
For example; a thicker Poly finish, as many factory guitars are, will allow the guitar to withstand greater temperature and humidity variations, without risking as much damage to the finish, and possibly the wood, when compared to Shellac or Nitro. However with a thicker finish, there's a possibility that the top doesn't vibrate as much, and has a potentially negative effect on the sound or volume. But more often than not, I've found this is all in the head, as beatiful music can be played on any guitar really.
In my experience, there are more timbral and dynamic range possibilities with thinner finishes whatever the type, but is this better? It's up to the player really to make the best sound. And there are more important variables related to tone such as wood and strings. Ultimately for me the finish is not that important in regards to the sound, it's just a protective coating.
That being said, I don't like Shellac- mostly because my sweat destroys it. I prefer Tru-Oil which is not as common or popular. Nitro cracks, and interestingly this is an expensive feature in "aged" or "relic'ed" electric guitars!
Drifting a bit, but Poly is good for campfire guitars or jam sessions or similar.
Also it might be important to consider that Nitrocellulose is a byproduct of the automotive industry, it's quite toxic, it has a unique smell (as do all finishes except Poly) and whether or not this is important to you as a guitar buyer.
"I can sense the atonal...I always try very hard to bring it out."
- Julian Bream