Well, yes, technically both Superman and "mild-mannered" Clark are disguises and there's a real persona underneath them both. But the question is, which one is closer to the real persona? Pre-Crisis, when Kal-El dropped the disguises and acted like his true self, he acted like Superman. In his free time, when he was by himself, he wore the cape and tights and hung out in the Fortress of Solitude conducting scientific or medical research to help the world or building new crimefighting tools or using his heat vision to carve Kryptonian writing into his giant diary or engaging in his creepy hobby of building statues and trophy rooms to honor all the significant people in his life (including a Clark Kent room so that visitors to the Fortress wouldn't get suspicious about the omission).
Christopher Reeve's version didn't go quite that far, but when he dropped the act, he may have called himself Clark, but he acted like Superman. Indeed, that's why his Superman is so engaging -- because he's so guileless. You sense that what you see is what you get. He's not acting tough and macho -- he doesn't have to. He's just a guy. A nice, neighborly guy who wants to lend a hand and happens to have powers and abilities far beyond those of mortal men.
By contrast, the post-Crisis version, when he's alone and free of artifice, acts more like Clark. He hangs out in his apartment, he wears street clothes, he does normal stuff. The only real difference between private Clark and public Clark is that the latter doesn't openly use superpowers. This is the version Dean Cain played in Lois and Clark. A large part of it is that he has more ties to his life as Clark in this version -- both his parents are still alive and available to hang around with, and since he's less nerdy as Clark, he has closer relationships with his co-workers, particularly Lois.
