Personality wise, which is most important, a captain should be a natural leader, his sheer presence on the pitch should signal confidence, vigor and determination, he should be able to inspire his teammates, especially in difficult situations.
Footballing wise, it makes sense to make the player with the most ball contacts captain IMHO, the player who controls the game, the mastermind. Usually, it's a midfielder.
That's why Gerrard would have made more sense IMHO, but that's just me.