We should also think about what most MLB caliber players do when they first enter the bigs. Many players, even all star caliber players struggle anywhere from a few weeks to a year or two until they really grow into the player they are.
I don't believe we have the ability to tell how fast a guy is going to hit the ground running at the big league level by evaluating his box score, and I think we've seen this time and time again over the years. We should think about that too.
Story is going to get first crack, whether or not people like that he is. If he comes back and is the player he was last year he very well may be just as good as Arias would be adjusting at the big league level. And if that is reality, then why wouldn't you take the extra year of control and play the guy you HAVE to pay.
If it looks bad with Story you can always call up Arias. But if you call him up first the clock starts ticking.