I'm usually in favor of giving the young players a chance, but I'm firmly on Willi Castro's side of things. Austin Martin has been a poor defender in the infield in his minor league career, and Castro can handle himself at several positions, and lately has looked good enough at 3B.
Martin isn't all that great of a hitter in the first place, he has a career .255 batting average in the minors with no power. Any random AAAA player can have a two week long hot streak in AAA, and even with that he's got a paltry .733 OPS in St. Paul. He'll take a lot of walks and probably get on base more than Castro, but we have enough negative value defenders on this team already. I need to see a lengthy stretch of time where he hits well at AAA to justify a call-up. This lineup needs hitters who can generate runs, and I don't think Martin will help accomplish that goal.