Given your rotation, and the number of Orioles in it (plus Lohse), I would try to retain Miller.
I get why you're trying to upgrade your OF, but I would be exhausting other options.
Just my 2 cents, though.
I tried to push for dealing my 1st overall pick or Brett Lawrie, but he's most interested in Miller (pitching prospects are incredibly valuable in this league). I tried looking at the deal as if I were the Cardinals, and if the Braves said they're making Heyward available, I'd consider sending Miller and Cox for him. I have high hopes that Zimmermann, Hughes, Matusz, Britton and then Carrasco/Litsch/Lohse would make a solid rotation. I sort of expect Stewart to be a reliever. I also have McNutt, Trevor May, Brad Lincoln, David Holmberg, Roman Mendez, and a hand full of other pitchers in my minors, so I should have some mid to back end of the rotation talent on the way.