Theoretically speaking, the metal cover can take more abuse. Although the dust cover isn't supposed to be a high wear or impact item.
On a DI gun, metal all the way due to heat issues and vent gasses coming into direct contact with the dust cover if it's closed and the weapon is fired.
On the 416, this isn't an issue and if the plastic cover holds up durability wise, what's the issue?
There are a lot of end users out there that beat the 416 like a red headed step child and I have yet to hear of a dust cover breaking. But then again, I don't know if the 416D ships with a steel cover or a plastic cover. A quick glance over the 416's imported into America show lots of steel dust covers. Did they ship this way or were they replaced by the end users?
Like I said, so far no complaints to be found.
AGR notes that the 416Ns come with polymer. How have they held up in the cold and in the sandbox so far?