+1 demo.
It has this design integrated into it so that it has a "static disc" thing. Don't know if its the actual name, but its purpose is to basically keep the rear rotor spinning at the same rate and angle when the suspension comes into play. Hard to describe without looking at it, but it is a really good design/function which the yakuza doesn't have. I think that there is a fair bit more technology gone into the demo 9 and for that reason alone I would sway towards that. I would arguably say that it is equally, if not more versatile than the yakuza.
on the other had go with your heart, and if you really like the yakuza for its own merrits, then go for that! I am 100% on the fence with the two of them. The only reason I would get the demo9 is that I think the design is slightly more intricate and it is much more of a hard core downhiller.