Nearly 16 million volumes of Kodansha’s Attack on Titan have been sold in Japan in the last twelve months, compared to 18.2 million volumes of One Piece, according to Oricon, via AnimeNewsNetwork.  That’s a remarkably small margin for One Piece, given that it’s the perennial best-seller there, with four million copy print runs of new volumes (see "Four Million 'One Piece'").  And there are over 70 volumes of One Piece, and only 11 volumes of Attack on Titan.

Sales on Attack on Titan have been growing rapidly in the U.S., especially in the book channel  (see "'Attack on Titan' Passed 'The Walking Dead'"), although obviously on a different scale than sales in Japan.