Honestly, I think Cason vs. Arrington is a grass is greener thing. Cason might be better on the outside, but Arrington is better in the slot. And with Dennard/FA#1 (be that Talib or someone else)/Dowling, we're looking more to fill a slot role.
I see where you are coming from, but if Arrington goes for the green (why wouldn't he?) then I see Cason as a good alternative. Actually, I think Arrington is at his ceiling, and Cason has some upside in a different system. Obviously, that's just my opinion.