The rolling radius of 10", 12" and 13" wheel/tyre combinations are virtually identical and those who say the 13" wheel give better speed/rpm are wrong.
The advantage of the 10" wheel is that it gives a deeper tyre sidewall depth which is better able to soak up bumps and irregularities in road surface.
The deeper the sidewall the smoother the ride will be. The Mini has very little suspension travel, so a deep sidewall is a big advantage.
On my road car I have 275/40 x 19 tyres on the back with 245/45 x 19 on the front and it really does feel poor road surfaces. I'm thinking of changing to 18" or even 17" wheels when these tyres are worn.
