I'd say try them both - whatever works best for you is the way to go. That's what the Holley tech reps used to recommend. The fact is you can successfully tune the engine to use either scheme.The "ported" vacuum will let your engine sit there and idle at a somewhat retarded advance setting - raising the water temperature (and exhaust thumping nicely). Running the idle advance on manifold vacuum will lean it out and raise the idle speed slightly - and cool off the water temp a bit too.