Henderson is beginning to remind me of Danny Murphy, who was obviously a gifted player. I do feel Henderson's defensive game needs a bit of work still, but I feel he does have natural leadership qualities. He is the more likelier of the two to be the captain. His passing is good, but again not a standout. He is one of those players that could improve everywhere and be exceptional, or he could improve marginally and be a good Premier League player.
Allen however is probably slightly better defensively, and his problems have lied in playing so far downfield, rather than a lack of quality to not be able to play there. That is something we'll hopefully see change with the implementation of Rodgers' rotating midfield, and will see him push further up the field. I think he is a good leader, albeit I can't see him driving the team on. I do see the value in having a player like him in midfield. His simple style actually really adheres itself well to building a team in the middle of the park, and I think his eye for a pass is often over-looked because so often he'll choose to retain the ball, than play a riskier pass than he feels is necessary...
For me, Allen edges it, but good post because I think it is close, I just feel most will say Henderson.