We tried the GXW earlier this season on our SuperStock 750, and found that it wasn't quite as durable as the ERV2, nor was it quite as free-moving. And, when the GXW's time was up, it stretched a lot in a very short amount of time; the chain was actually dragging on the belly pan after one practice session! Also, replacement GXW master links are $15 each, instead of $3 for the ERV2. Of course, this is only an issue if you are constantly swapping gearing and need to use differnt-length chains to keep the tire off the swingarm.
In the end, the extra 10% in price for the ERV2 was more than offset by the 25% longer life and the additional confidence that the chain would last the entire race without unraveling.