I think your best bet is if you can look over the individual rifle you are considering buying, as opposed to assuming anything based on generalities.
Bingo
What to look for on any m14s/m305 , poly or norc
Is the flash hider correctly machined? Look for a tilted front sight base
Is the rear sight base centerline machined to agree with aperture center?
Headspace varies so if you can check it do so
Any other issues can be easily adressed
Soft metal in the rear sight can not be established until the sight has been removed for inspection
I prefer the grey parkerizing of the norcs over Poly's as it is definately a tougher finish.
In my opinion there is no advantage of buying one over the other with the exception of headspace which in the many poly's I've worked on has been a few thou closer to NATO spec than the recent norcs
A surprising number of my clients who purchased plastic stock poly's are having me customize usgi fiberglass to ditch the poly plastic
One poly stock I've examined after one clients range day, exhibited heat damage in the forstock which I assume was caused by hot gasses from the gas system.