I need a 14m since I ride in the ocean, twintip, current and waves, and weigh 175 lbs (80kg). Most of the time I ride my 14m (different brand) as the wind is normally between 13 knots and 19 knots. I then use my 11m when the wind is between 18 and 25 knots. The wind rarely get's over 25 knots, but when it does, it would be nice to have a kite that would ride 25-30 knots.
So I am on the fence, 14/10/8 or 14/12/9. Buying kites really sucks, I think it's worse than buying a car.
I'd go with 14/10/8.
I have a 14/10/6 quiver of Rallys (I know, different kite, but same company) and the jump from 10 to 6 is too much AND the small Slingshot kites depower so well that the 8M will probably take you as high as you'd want to go.
The 10 will be your money kite, and you've already said you want to have the big boy (14) for the light days.
I'd throw out one option for you if you are doing a Slingshot quiver...make your 14M the Rally, and make the 10/8 the RPM
probably excels in the smaller sizes but on the large size
I can't think of a Slingshot kite better than the 14 Rally. Fast, fun, great range, etc. etc...