Marine, I think you've nailed the problem with your post. I don't view the two guns as substitutes for each other. As you suggest, the purposes probably vary with the quarry/game. I like your thinking on the Fox and other gun combination as it is quite doable. There is much versatility in the plan to give the edge to the Fox.
Another consideration, in my opinion only, is that the newer Foxes have a superior fit and finish while the older 21s are just fine the way they are. So, if you want to end up with one of both as shooters, I'd go with the brand new CSM Fox and recover a 21 to fill your clays needs.
That being said, you probably can't go wrong here.
