I have a Holley 3x2 set-up and I have a port vacuum outlet on the main Holley.  I can provide either port or pure manifold vacuum to the distributor.

The difference is port vacuum disappears at idle.  You only have vacuum advance at cruise.  I have researched this and cannot find any recommendations.  As an afterthought, I don't see many port vacuum outlets on more modern carburetors.   I am comfortable with manifold vacuum sine that is what I have used on previous engines.

Any opinions??