Initial startup is where a sensored makes the most difference since its using the actual revolutions versus the sensorless using the em field. The em field is weak at lower revs, and so there has to be enough of a em field to get the sensorless esc to "see" the sensorless motor.
Once it does however, from what I understand a sensorless is more efficient, and can potentially reach higher rpms. Hence why you see many dual mode sensor/sensorless setups.
Personally, Im hoping Tekin comes out with a Mini RS the same size as the B1.