DR Michael S - the early albums were originally released in mono, therefore some consider the sound more authentic. But the mono box set doesn't include the later albums. Also, some different takes used on the mono mixes versus the stereo. As a compromise, some buy the mono set and supplement with items from the stereo set, which are available individually.
Some reviewers claim that which mix is better is really on a song-by-song basis. The stereo box has deluxe packaging [more extensive liner notes, mini-documentaries on each album, etc.]