Never owned or used a Win 21. So I won't compare one to an RBL. Many opinions surface for the RBL, good and bad. Most the bad seem have more to do with CSMC than the guns!
What is so vastly superior about a Win. 21? Better steel? Nicer finish? Big name? The few I've looked over felt sort of clunky for gauge and the finish didn't seem to be that great for the cost. This is not a knock on Win., if a M21 winds your watch so be it! I had a M12 in 16Ga that I regret not keeping, a great gun in all respects.

Chief