Personally I prefer the F-brush and if you are using amp steps on the dyno I like to look at the 28-30 amp number, at least for 6 cell touring car.
Would it be possible to have two sparate part numbers for each motor? Maybe one version tuned more for the 18-20 amp number and the other for 28-30 amp. That way if you are running 4 cell in very light car you could get the 18-20 amp motor and if you are running 6 cell in a heavier motor you could go for the 28-30 amp motor.
I'm not sure how hard it would be to do something like that but I figured I'd throw it out there.
Oh yeah, I agree with Stephen on the dyno stepping, I think torque steps are just easier to compare.