I've ridden 12's and 9's and although the 9 has a little less displacement(and power) it still has gobs of torque and respectable hp. Due to the lower gearing in the 9 it accelerates basically just as fast too. You just lose out a little by having slightly higher highway cruising rpms.
I got the 9 and was hesitant, wanting a 12, but having owned it for 2 years I have no regrets. I'd buy another without issue, they are basically identical. I don't even know why they made 2 different bikes.