BTW - I had two 21.5 motors to hand and did some very rough and ready measurements. One had inductance measurements of approx 30uH and the other 22uH. Measurements also showed that inductance changed with the position of the rotor. They had the same static resistance. The one with the lower inductance performs much better on track.
The 13.5 I checked had a measurement of approx 9uH.
These measurements were not done with expensive test gear, but I think they're reasonable.