If the roster allows it, I'd much rather keep Polanco over Kepler as year-in-year-out Polanco is the much more reliable hitter. While the Twins have several good hitters, they lack reliable hitters, and I think that unpredictability is the main cause of the lengthy slumps this team has seen the last several years.
Still, I think if they trade for top end starting pitching, they can use most of the free agent cash to sign a reliable bat making either or both more expendable.