Tuff call old chum
Depends what you are after and what your level is, what board ya got etc..
The 10.5 is certainly the choice lighter wind kite around at the moment imho. Not only does she go like stink in 10 knots but its light and fast to work and boost.
If I could have only one serious spec kite at the moment then this would be it for UK conditions, complimented with a cheap, used high wind kite ( small 30 knot+ kites are much of a muchness ). I have used most of the 2002 kites and last year flew Airblast as choice.
Having said that, the 8.5 is the real peach of the skoop range... its just not going to entertain much in less than 13knots though I've not tested the low end of the 8.5 yet and the 6.5 got me rockin in 15/16 knots onshore so it may well surprise yet?