Pros & Cons of vacuum vs. mechanical secondaries?

My car (well leave out the brand for the moment) currently has a very mild truck big block in it. Current carb is a 600 afb style edelbrock. Vacuum actuated secondaries. It probably needs a 750 vacuum holley or eddy.
When it gets the 496 or 540, an 850 dp will be the minimum carb, it will probably get a 1050 dominator.
My street strip 440, has a 850 dp, would not even consider a vs.
As for factors that determine choice, I would list rearend gear first. 2.76s won't like a double pumper much.