For those of you that are curious to know,
I dropped by my MINI dealer and took the measurement of both the 2-spoke and 3-spoke steering wheel.
Both are exactly the same width - 14.5 inches.
The 3-spoke 'looks' smaller by virtue of the centre hub being much bigger than the 2-spoke, thus giving the steering wheel the illusion of being smaller.
Also, the 2-spoke is clad with perforated leather, while the 3-spoke is not. From my experience, perforated leather feels and grips better, but you may disagree with that