[Mr 5 0]

Unless you are running a big-block race engine, vacuum secondaries (using engine vacuum to pull the secondary throttle plate open) are the best way to go - which is why most aftermarket 'street' carbs are equipped with them. They are more practical for the street. However, 'bigger' carbs (750, 850 CFM) intended for big-block engines and/or race applications (usually with very high-lift cams) can be bought with mechanical secondaries if you need them.

Holley sells a secondary diaphragm spring kit that allows you to adjust the secondary opening rate, if you wish. In fact, Holley sells just about every part of their carbs except the body, a la carte. Again, check the Holley website for details.