The W3v3 line was Engineered after the W6 and W7 lines. A lot of the patented technology from the W6 & W7 lines was applied to the W3v3 such as the elevated frame cooling, VRC, new surround attach method etc etc etc. So basically the v3 products are more technologically advanced and are more efficient in overall terms.
I personally have one 15W3v2 and i Love it. I can push it to the point where you'd expect to hear the kiss of death and nothing. It outputs huge amounts of clean undistorted bass. It is not as articulate as the 13w7 but it is definitely a man's speaker, especially in a sealed box.
What speaker are you looking to get?