which works better

That's debateable. The system Mercedes uses is extremely limited. It only works in certain cities during a certain time of the day on certain roads that are below certain speed limit. FSD work anywhere in the US even on roads that have not been mapped.

Drivers can activate Mercedes’s technology, called Drive Pilot, when certain conditions are met, including in heavy traffic jams, during the daytime, on spec ific California and Nevada freeways, and when the car is traveling less than 40 mph. Drivers can focus on other activities until the vehicle alerts them to resume control. The technology does not work on roads that haven’t been pre-approved by Mercedes, including on freeways in other states.