I love the fact that we're debating options where none seem to be a case of "well we need *some* alternative choice". I'd be happy if we named co-captains. No player plays in every game, so knowing who the other option is ahead of time allows them to develop the leadership between them.
Think back a few years, and our only option really was Robbo. Yes we tried DC and Marler while Robbo had the England captaincy, but neither of them really suited the role (although with Marler, I think than now he's started to address his demons and has matured a bit he's a much better leader on the pitch. Horwill was a great choice while fit, but it's now good to have decent options for the next generation.