Lets see, even though the themes are similar, each theme is very unique. I personally think the Star Wars theme is overplayed and overrated , I like the medley of themes that make it up (though you could legitimately say the same thing about almost every other John Williams' themes), however what makes Superman cool in the concert version, that is, is the brass intro that hearkens to the same emotions as a Fanfare for the Common Man by Aaron Copland or Sprach Zarathustra by Richard Strauss (aka 2001: Space Odyssey theme).
Also, the last musical bars of the Star Wars theme could be very annoying (The slow whining strings of the Rebel Fanfare demonstrate how the rebel fanfare just SHOULDN'T BE PERFORMED THAT WAY!) while the ending of Superman is more slick sounding, albeit rushed.
So to grade theme together would be a mistake, but Superman is a refreshing change of pace, while Star Wars is ingrained in the pop culture, thus canceling out Superman and some of his lesser known themes.