I use ported for my 383. In my opinion you only want to go with a manifold source if you have a large cam and have trouble getting the engine to idle decent. If you have a mild cam and go to manifold vac you can wind up turning the idle screw down so low to get a normal idle speed that you get surging at idle - the extra advance at idle makes idle speed go up and you wind up trying to compensate by turning the idle screw down.
The ONLY difference is at idle. A ported source will have no vacuum at idle and no vacuum at full throttle, so the only time vacuum advance is added is at part throttle. Manifold vacuum pulls at both at idle AND at part throttle, with no vacuum at WOT.
Hope this helps.
68 Camaro with 383 small block. ET - 11.9's at 112 mph and never trailered.
10.3:1 KB pistons, Eagle cast crank, 5.7" SIR rods.
Trick Flow 23* heads ported 2.055 intake, 1.6 exhaust.
Comp Cams 282s mechanical cam with 1.6 rockers.
Victor Jr intake and Holley O-3310 carb.
1-3/4" Hedman Headers, full length Flowmaster exhaust.
TH350 trans, 3000 stall, 3.55 gear.
CE subframe connectors, drag shocks, SSM lift bars.