More durable (and lighter, and cheaper) = better for a carry gun that will be getting constantly exposed to sweat, humidity, rain, hot temps along with the above, etc...
Lighter? Check.
Cheaper? Check.
More Durable? Only in terms of corrosion resistance, and only compared to bare/untreated metals of certain types. Long chain polymers have amazing strength for their weight, but pale in comparison to metal alloys, especially high carbon steels. Polymers do not have good abrasion resistance (especially without lubrication), cannot withstand high temperatures, are degraded or destroyed by many chemicals, and, as mentioned, deteriorate from UV exposure. Steel can withstand very high temperature, is far more abrasion resistant, is completely unaffected by UV, and the only chemicals that can affect it are corrosive acids, and they act slowly.
Dunk your 1911 in brake cleaner for a couple hours, it will look lighter in color after it dries, until you wipe it with a silocon cloth. Dunk your polymer gun in brake clean for a couple hours, it will be all gooey and completely useless.
There is also a reason slides and barrels are still made of steel, and that polymer framed handguns use steel or alloy inserts for the frame rails.
Buy a polymer gun because it is lighter. Buy a polymer gun because it is cheaper. Buy a polymer gun because it feels better in your hand. But for Pete's sake, don't buy a polymer gun because you believe it is stronger/tougher/more durable than metal; It isn't. And all polymer guns still have plenty of metal parts, so it is foolish to think that a polymer frame = gun 100% impervious to corrosion.