Blackbeard has never been specifically shown to be interested in Poneglyphs. Nor has he shown any inclination in trying to get to Raftel. Therefore one can only conclude that his idea of becoming the Pirate King is different from Luffy's. He is not really interested in One Piece, but wants to become Pirate King through world domination.
He and his crew will at some point single-handedly defeat everyone (except for the Straw Hat Pirates), from the Yonkous Luffy hasn't yet defeated to the WG, and the Marines too, thus conquering the world. Luffy will then challenge his supremacy when he's back from the only island Black Beard hasn't managed to conquer, and with what he has discovered on Raftel, the SHP will defeat the Black Beard Pirates.
Basically what distinguishes BB from Luffy is that one wants to be A Pirate-King, i.e a king who happens to be a pirate, and the other aspires to become THE Pirate King, i.e the king of pirates.