While you have more knowledge and immersion into the fandom, I have mere observational skills. Sometimes that is all it takes.
That said, while The Cap'n is well... blatant, Superman is more "subjective" in opinion.
American Invincibility, Good guy always wins (mind you, I realize that isn't always the case in the comics/movies/cartoons/etc.), color scheme and style... it screams America. To me, an outsider.
As such, I can only go off of what I see right in front of me and what little experience I do have. Which is probably more than you might assume, given I didn't grow up in Japan or grow up during the Toonami phase or whatever and actually have more experience with American Cartoons than Japanese Anime, for example.
Just my two cents really. They both scream America. No, not the real one. The imaginary one in every hardcore Patriot's mind. Yeah, that one.