W6's go deeper, but they dont have nearly as good SQ
I have w3's and i like them a lot
I think they go down to like 25 hz well but the w6's go down to 21 sounding good...i personaly dont think tha 4hz matters unless you listen to bass mechanixs for fun, like i do. And the W3's sound a lot nicer, and they are marketed more regularly than w6's
the extra money for the w6 is a lil pretty sticker, a slightly larger mag, and the number 6
Its your call though