Well the reason I chose the polymer lower receiver, in my own opinion I think it's the best.
It's dirt cheap.
It's stronger than all others.
Yes it's ugly though.
It weighs less than most .
It will never scratch up.
Honestly I think it's the best all around.
The lower receiver doesn't take much abuse anyways I think when it comes to building a ar platform rifle that the best quality parts should be invested in the parts that take abuse, thus being the barrel, upper receiver and bolt carrier group.