In a heat torture test, the metal will outlast the polymer. That said, I have some of each and I'm fine with the polymer really. It's been around for a while and is pretty durable.
I mean, If I'm in some sh.it where I'm burning down my polymer handguards, chances are I'm not coming out alive anyways. Hell, I doubt I would even be carrying enough ammo to really burn it down like that.
Back in El Salvador we had A1's. We got them pretty darn hot in some training ranges doing full auto stuff. So hot you could barely hold onto them. They lived through it just fine. Old school triangle handguards.
