I run polymer both, depending which one locks better in which firearm. Polymer tends t work better in my RPR, while my KRG Bravo prefers the metal ones. Overall both metal and polymer function in both rifles, but some just lock up a bit better.
Assuming these are the AICS mags that I think most of us use, there are a few actions that don't work as well with the poly mags than they do with the steel ones. AICS was originally steel, so thicker feed lips or different dimensions in other areas can affect their function.
They both work well. It’s often dependent on the rifle at hand. My Steyrs feed great with the polymer mags they were designed around, and my CZ 557 UCS feeds very well with its proprietary steel mags.
I like Lancers the best and have always worked great for me in all of my ar15/10s. Steel where you need it and poly where I like it. That being said buy them all including aluminum because anti-ass wipes are constantly going after them.