I'm having a discussion on another forum about this topic. My basic view and understanding is this. Ported distributor vacuum advance was a result of early 70's emission equipment. It comes off the venturi above the throttle plates and is effectively isolated by the throttle plates from manifold vacuum, more specifically the throttle shaft as the defining point much like an expansion valve in a refrigeration circuit. As such it is my understanding that the amount of ported vacuum is going to be proportional to the amount of air going through the venturi and not in anyway effected or controled by manifold vacuum below the throttle shaft or amount of the throttle being opended.
I personally prefer manifold vacuum because to can keep the throttle plates closed more at idle making idle mixture more precise or mre effective along with vacuum advance backing off on total timing under higher engine loads while ported can't do this. I think if some one is going to run ported instead, they need to know exactly the advance rate vs engine load and set up accordingly. I found a post by a GM engineer who posted on a Corvette forum on this topic. I think my understanding was pretty close to what he stated except he did not go into more about ported as the throttle is opened, only what it was doing at idle. I tried to paste what he said last night on a post but had some trouble and wasn't able to post. I'll see if I can do it today.