Yes, both are for a clear coat shine. Poly is essentially a spray "plastic". It cures hard, buffs well, and can be drop filled with crazy glue. Some people prefer nitocellulose because it allows the wood to continue to age without fully sealing it, but it cracks over time and reacts poorly with rubber. As another example, nitro is what they used to finish cars in the old days, which is why you had to keep it out if the sun, away from salt and wax regularly. Car manufacturers no longer use it, and neither do I. It is also bad for the environment, another reason it fell out of use. IMO, nitro is for those who believe in tone wood, since the wood aging (especially when played), is supposed to improve the resonance, which is very true for acoustics, but often debated for electrics.