Basically, for the average Joe the Spit 8 accelerates slightly quicker, turns slightly better, climbs slightly better, has another 10-12 mins of time of flight, and has far more "oomph" in WEP than the Spit 9. However, do not count the Spit 9 out. The Spit 9's biggest advantage over the Spit 8 is it's stability. The Spit 9 is more forgiving and a player can "hold the edge" easier because the Spit 8 will dip that left wing quicker. Also, if the fight turns in to a marathon and WEP is not a factor the Spit 9 is slightly faster than the Spit 8 with mil power.
The Spit 8 is more versatile, too. It can carry a 500lb bomb and the Spit 9 cant. The range of the Spit 8 allows it to be a better escort fighter as well. I like them both, and because I've sworn off the easy mode Spit 16 it is usually one of those 2 Spits I take to the virtual skies with.