Ther's a lot to be said for Holley double-pump mecanical secondary carbs. There is a seperate, full-size fuel supply for the secondard bowl assuring that extended secondary use won't result in fuel starvation. Because there's two pumps and more positive and sooner opening secondaries, you don't have gas hangin in the rear floatbowl till it gets stale. You also usually get a real rear metering block, which makes tuning the rear as familiar as tuning the front. They are the better choice for an engine that has poor vacuum.
Having said all that, I've got a couple of vacuum secondary holleys that work fine. I don't really save any gas with them but I suppose the potential is there