If I had to bet on better defensive value going forward, I would choose Mountcastle over Stewart. I would think that Mountcastle can play an average to slightly below average LF and won't embarass himself. His best position is probably 1B and his athleticism would be an asset there. There have been many worse corner OFs for the O's, Delmon Young, Trumbo, and of course Jack Cust come to mind.
I'm predicting big things for Lowther. He might not have the most impressive pitch arsenal, but he knows how to pitch.. There are a lot of above average starters who don't throw 95+ and a lot of guys with electric arms who never have a major league career. I wouldn't be surprised to see him have the best career of this crop of O's pitching prospects.