ZX-6R:
-111 hp (81.0) @ 12,500rpm / 66 lbs torque
-379.2 pounds

ZX-9R:
-143 hp (104.4) @ 11000 / 101 lbs torque
-407.9 pounds

Bigger bike: more power, but heavier
Smaller bike: less power, but lighter and handles better

Personally, I like smaller bikes that have average power, but handle really good. Im not a big guy, so I dont really have a desire to get a 1000cc bike.

BTW, the 1/4 mile times of both bikes are almost identical. They differ at most only by a second. The main difference in performance is in the top end. Of course the 600 has to struggle to hang with the 900 when doing 150+.