I've ridden both, and I'd choose the V-10 over the Demo. No question about it. yes, it is personal preference, but my experience has been that the V-10 is a far more enjoyable bike. The Demo's front end feels super nice going through steep rock gardens, but in the corners (where a lot of DH speed is gained or lost), the rear end feels really sluggish. To me, the V-10 is just about as adept in the front end, with a much lighter feeling rear end. It feels like when I push into the V-10 needing some rebound/pick-up, I get something back. In contrast, the Demo seems to soak up the push, robbing me of the push and giving nothing back. The final deciding factor was when I happened to run into a few reps for Santa Cruz at a trailhead, having a dealer-only ride event. I'm not a dealer, I'm just a nobody. But they offered to set up a V-10 carbon for me and let me take it out for a ride. Fantastic bike. It sprinted better than any other DH rig I've ridden, and it was liveable pedaling up milder climbs. Note, I said liveable, not enjoyable. It was still rather sucky. But the bike ate everything in its path, yet felt light and nimble.
If you're worried about durability, check out the video of the Santa Cruz test lab. Their bikes are strong. Very strong.