That's a very hard call and I'm glad I don't have to make that decision.
I do believe the captaincy should go to one of the 'younger' players, however. There's no point picking a captain who clearly won't be around for the next RWC and England have nothing to lose and plenty to gain by being bold now and taking advantage of some of the newer blood. Although he's yet to prove himself internationally I reckon Owen Farrell (Saracens) could be one to watch and a future captain.