8 is a bit heavier due to the retractable tail wheel and the two leading edge fuel tanks. It also has the larger rudder. In terms of turning, I think it would depend on what alt the fight was at, as the Spit IX engine we have is the Merlin 61 geared for more high altitude flying and the VIII has the Merlin 66 which was best suited for medium to low alt flying. Later IXs also had this same engine. The XVI has the American made Packard Merlin 266 which is the same as the Rolls Royce made Merlin 66.
Personally I'd take the VIII over the IX because of the better engine suited to the AH altitudes where there fights. In the end I'd say the winner of a VIII vs IX fight would be determined by the pilot, not the plane.