the difference is minimal, but basically a sensorless one just shoves the pulses through the motor, while the sensor one has a sensor to find out where the rotor is, and adjusts the pulses to it's rotation. basically, a sensorless one is cheaper and snappier.
The biggest reason for the sensored brushless setups, was that early sensorless systems were very prone to cogging (uneven power delivery). Sensorless systems use back emf to determine rotation and position of the rotor. If pulses are delivered at the wrong time (out of phase) the motor will not be efficient or smooth (cogging).
Sensored systems get around this limitation by knowing where the rotor position is at all times. Even with the newer sensorless systems you can get momentary hesitation especially if you inadvertently loose traction coming into a turn, essentially backing into a turn, when the throttle is applied to power out the the turn it can hesitate slightly. (At least my mamba max setup would)
That scenario does not really cause a problem though, because the delay is very small and you have already lost more time from the mistake anyway.
I have the worst luck with motors/engines and esc's.
If it can cog, I've done it. I will never, ever run a pure sensorless setup ever again. a sensorless setup will run absolutely perfect, until your main comes up in the biggest race of the year. Then it'll accidentally go backwards at the buzzer and collect the entire field in a big pileup.
The software in the esc can make up the difference between smooth and coggy. The Tekins and MMP can get a bad version loaded and act crazy. Thats the fun of flash memory. Even sensored stuff can cogg realy bad if the software has a glitch. We're asking a whole lot from todays Esc's, it's not easy for them to do what they do.
With my Tekin RS the sensored operation seems to give more punch down low. It is used for 21.5, 17.5, 13.5, 10.5 motors. It also gives the Esc different features that are nice to have with slower motors.
In dual mode, it uses sensored operation at low speed , and goes to sensorless as soon as it can. This seems smoother and more precise, a bit wimpy. It's commonly used for 8.5 and faster motors. I don't have much experience with it, I'm currently running sensored only for all my cars.