Ohio Dan is right on the money. I am shocked your engine builder didn't know this (or maybe you mis-understood him).
Judging by your cam, it looks like you are more concerned with performance. Basic rule of thumb, heavy cars work well with a vacuum secondaries, auto cars typically work well with vacuum secondaries, light cars or stick cars work best with mechanical secondaries. This basic rule applies to street only. If you want all out performance and don't care about mileage I say go with a double pumper (mechanical secondary). What size engine are we working with? Why would you compare a 750 VS to a 650 DP? True with vacuum secondaries they are more forgiving if you over carb but, if this is a 406 you are working with you need at least a 750 (maybe more depending on the planned use of the car).