One is full manifold vacuum & the other is called ported vacuum. The ported vacumm has no vacuum at idle. Most of the time its best to use full vacuum for the vacuum advance, but sometimes the ported works better. If it surges & idles rough at idle the ported may work better. If its an auto trans. & it clunks or lurches when you put it in drive or reverse sometimes the ported will work better after setting the idle speed down. Its a matter of experimentation.
