Well, as soulsaver mentioned, the W3 build quality is better and it looks better (brushed aluminum, etc.), but the x600 is quite a big drop from the 6600 in the Z71v, so be sure you won't need as much graphics power if you decide to do it.
Feature-wise, the Z71V has better speakers, better sound through the audio-out, and more USB ports. Both the W3v & Z71v have a modular bay and similar battery life.
personally, I wouldn't do it. But I also don't want to give up my graphics card for anything less powerful, which is a good part of my reasoning.