[Tommy]

I run a single Holley four barrel with mechanical secondaries on my 410 ci custom built Windsor. It is a race setup with no choke. ... I'm told that vacuum secondaries compensate for overly rapid throttle opening by delaying the opening of the secondaries until the vacuum signal says conditions are right. The pro is that it prevents an overly aggressive driver from dumping a lot of fuel into the engines from the secondary power valves before the air flow from rising RPM can catch up. The con is that there is a slight delay in when the secondaries open that is not under the direct control of the driver. The optimum for race applications is mechanical secondaries AND a driver who can match the opening of the throttle with engine RPM and conditions.

I've driven mine so long I don't think about or notice the opening of the secondaries. And I don't have enough traction below 80 MPH to abruptly give it full throttle without swapping ends. So it has not been an issue for me. If you like to abruptly go to full throttle without regard to engine RPM, then vacuum secondaries might be a better choice. But if you seldom abruptly go to full throttle, it probably doesn't make much difference which you choose.