"armor" makes more sense than spandex because of the default; spandex makes no sense. none of it makes sense, really. why would superman have a cape, for crying out loud. so, really, we're debating silly upon silly.
but, to me, something other than the comic book cliche of pajamas just makes more sense, even if the only justification is there's no reason for the alternative stretchy cloth so-skin-tight you can see sinew. if we're playing the "superman is invulnerable" card, then he should go the watchmen route and not wear anything...
But Superman started the "pajamas" cliche in comic books. So if there's a character who earned the right to keep wearing tights it's him.
Furthermore, from what I understand, he wears the costume because of the idea that it makes him more approachable despite being an omnipowered alien. The theatrical costume in primary colors keeps people from thinking of him as an alien. It also distracts from the fact he doesn't wear a mask and lets people think he doesn't even have a secret ID.
