They're two bald men with strong jaws...beyond that, they stand out quite a bit to my eye.
Lex's jaw's more square, where Clark's angles back quite a bit, as it's heading for his ear; Clark's got a noticeably cleft chin, where Lex's is smooth; Clark's actually got a neck, whereas Lex's flows pretty straight into his head; and Clark's got quite distinct cheek bones, but Lex's sort of blend.