You could use both.
But, I now only use the vacuum gauge.
The formulation of gasolines make the original timing marks only close/adequate. Moreover, we often do not know what vehicle our flywheels came off of. The timing marks on the flywheels (for the timing lights) might not be correct for the engine and peripherals.
Besides, the vacuum gauge is quicker and easier (and allows for a quick adjustment of the carburetor/s).
