What Does Superman Tell Us About Captain America?

Superman and Captain America share a lot: a reputation as the respective boy scouts of their universes, a sense of style, apparently a shared curiosity over whatever it is that's happening just over your shoulder. But what do all those similarities mean?

After reading the review of Captain America 2, commenter Cartagia suggests that those similarities only throw the different worlds the two characters occupy into sharper focus:

Cap is a terrific mirror for Superman. Where Superman strives for the American ideal, so does Cap.

But, in terms of storytelling, Superman is usually a loner surrounded by bland characters (with occasional flashes of brilliance), Cap is surrounded by morally grey people, and that is his real conflict. Not fighting against the enemy, but as CJA so brilliantly said "[T]hat's Captain America's true superpower — he makes things simpler, for everybody."

What do you think? Does Superman serve as a mirror to Captain America? And what do the similarities and differences between the two tell you about the different worlds they're living in?