They are two ends of the same chain, not a choice between two styles.

Specific RPMs makes your LEDs mirror the real car's rev lights. They activate at the same RPM thresholds as that car's own dashboard, so an LMP2 and a GT3 shift at visibly different points, exactly as they do in the car. This is the setting most people want.

Generic RPMs is the fallback. It is used for any car that provides no rev lights of its own, so you are never left with a dead bar.

The priority runs: Specific RPMs where the car provides them, Generic RPMs where it does not.

The plugin tells you which is in play. With Specific RPMs on, the generic section reads "Fallback, used only for cars that provide no rev lights of their own". With it off, it reads "In use for every car".

Specific RPMs requires a Pit Crew membership or above. If you have subscribed and it is still unavailable, activate the plugin on the plugin's home page while signed in.

There is also a Toggle Specific RPMs hotkey so you can switch between them without leaving the car.