The main advantage my 5D MK III has over my 5D MK II's is the better autofocus and performance in extreme high ISO's over 6400. I use it at up to ISO 25600 when I have to, and my 5D MK II up to 6400. Obviously, you don't want to push it to those values unless you have no choice, but in theater photography, its sometimes just necessary.
Its a pretty fine distinction, and irrelevant for most users, but, if you need the AF capabilities or the extreme high ISO capabilities, then its the one to get. It also finally has auto ISO in manual, but no exposure compensation, so full manual might be better in difficult lighting.