Which is better is going to vary somewhat by district. So part of the decision is finding out what a  magistrate judge is allowed to do in your district. Magistrate judges are appointed by the district court judges and the district judges determine the scope of what the magistrate judge may do. Maybe the discussion of federal magistrate judges found here will help.  In the district where I do most of my work the chief advantage of the magistrate judge is that the cases are brought to conclusion faster than the district court.