That's like comparing apples and coconut.
BL is brushless, unlike the older tech we used which were brushed motors (BR).
SL is for sensorless. This compares to sensored (no abbrev. that I'm aware of). A sensored motor and esc can locate the position of the rotor to ensure smooth operation. A sensorless motor and esc combo simply forces power into the motor and if the rotor is not in the correct position it may stutter (referred to as cogging) before accelerating away smoothly. Organized racing uses sensored systems almost exclusively.
As for which has the higher performance, you will likely get different answers from different people. Really it doesn't matter. With either sensored or SL systems you can readily buy something that will completely overpower your vehicle and/or cook the electronics if not setup properly.
If you ever plan on racing you should visit the nearest track and talk to the people there before you potentially waste money on something you cannot use. If you don't plan on visiting the track a sensorless system is an affordable way to go fast.