with regards to the difference between nitrocellulose and polyurethane paint, there probably isnt one. im not an expert on paint, but i have restored a few classic sports cars, so i know a little.
as far as i know polyurethane is just a more modern version of nitrocellulose. they are both applied as thin layers of the base colour, followed by layers of clear laquer. polyurethane, being more modern, is tougher,more resistant to uv rays and less hazardous to use.
although they are different chemically, i dont reckon you would hear any difference.