hmmm... nice query stuart.
IMO, whoever is the captain regardless of whether he is a defender, midfielder, or striker, doesn't affect the defending or the scoring. That would depend on the mentality set up i think. However, the captain obviously is the one who influences or inspires his teammates in their style of play.
If two team mates have the same influence value, i consider who is the more established player in the team and then his determination. For example, i'm play man utd, G.Neville is default captain. I signed on a free Friedrich and Metzelder who both have 19 in influence. When Neville doesn't play, I put Vidic as my captain having an influence of 15 even though both Friedrich and Metzelder is playing alongside with him because he is a more established player and has higher determination. Hope that helps!