Your distributor has two ways to advance spark timing. A set of weights that cause the spark to advance (occure earlier) this is strictly a function of engine speed or RPM and a vacuum advance that depemds on engine load. More load on the engine gives less or no vacuum advance.The reason for vacuum advance is to give better gas milage.
You can use either pure manifold vacuum or carb ( ported ) vacuum to send an engine load signal to your distributor. They both work and for your purposes both will be about the same result. Pete