Polmer 870 trigger plate assemblies are fine. I have them in all my personal 870's. buying a P model trigger plate assembly is a waste of money and yes I sell factory new ones if you don't want to take good advise and buy one.
There is a video from Ruger on my web site www.aiptactical.com
it is on the "Build your weapon" page about half way down. It is a drop test on a polmer trigger guard and on a cast metal one. Watch it then decide.
Other then that all the parts are the same in both assemblies except one $3 spring.