I think Munoz has the chance to do some really special things for this organization. He nearly made the club last season, after completely dominating in winterball. He's made a seamless conversion to Starting, even though it was initially supposed to be so he wouldn't rely on his curve as much. His success as a starter only increases his value to the Organization. He should be fixture of the Sox Pitching staff for years to come, in whatever role they choose for him.
Grilli is a full 3 years older than Garland, and is a reclaimation project after being injured and then cast off from two separate organizations. He's coming of injuries/surgery, and doesn't have the same "potential" he once did. Garland at only 24 years old, in his 4th full year in the bigs, who was once the youngest pitcher in the majors, clearly still has more upside.