The Miz doesn't really say or do anything original, but its how he does it. When The Miz calls someone out he's usually more threatening about it and is built up as a guy who can pose a challenge. Heath Slater, on the other hand, is put over as a walking joke, pretty much the total opposite of Ryback. When he calls out these legends, we all understand he's going to have his face kicked in (I have no problem with this either, nothing new to wrestling).
Slater is a comedy act, whereas The Miz, at least relatively speaking, is a legitimate contender. They are totally different acts.