Well, a couple things:
Marquis has never been (nor ever will be) an All-Star. Since Josh has, that automatically raises the perception of him.
Also, last season Josh Howard had one of the worst seasons of his career when he averaged around 12 ppg etc. Marquis best season ever was the one before he came to the C's when he averaged around 13 ppg. Clearly, the ceiling for each player seems to be drastically different. However, how good they are relative to each other at this point in their careers is a different matter (we don't really know for sure).
Also, if I'm not mistaken Josh Howard at least has something of a jump shot - though not a great one. Marquis has practically no jump shot to speak of. He doesn't spread the floor at all; he's a pure slasher / post-up player. The last couple of seasons, the Celtics have desperately missed a wing player with good size and defensive ability who can actually hit an open jump shot (basically what Posey gave them).