I'm getting ready to write some nice control software for my Lego motors and have done some initial tests to see how they perform.
For real use normally one would use rpm regulated mode of course.
Unless you need the ultimate rpm, the large motor is a more consistent performer.
The large motor is a good performer, starting to rotate at a low duty cycle, and very linear in its behaviour for both positive and negative duty cycles, it starts moving at 6% and has maxes out at 136 rpm.
The medium motor isn't so good in this respect, it doesn't start moving reliably until 30% duty cycle. It maxes out at 227 rpm in reverse, and 212 rpm forwards.