A 2-2 draw, which is possibly predictable given that these are two cars from the same manufacturer just a year apart, and both delivering a WDC.
However - my deciding vote goes to the RB8.
Why? - The RB9 was dominant, but only as a result of the tyre compound (IMO).
Whereas the RB8's success was down to the car alone, and the work that the team put in to make it a success. It had competition, and overcame that to win.
My vote is based on how the cars finished their development - one of them was a result of tyres, the other a result of design.
And the RB8 had a better livery