There's a good article over on Hot Rod that discusses pros and cons of the Holley 94 versus the Stromberg 97. Vacuum driven power circuits can cause the engine to run rich if the valves open too soon. On the other hand mechanical power valves typically don't open until the throttle plate is tipped in significantly - which doesn't happen often when one is running a dual on the street. The modern 94 replica's from Edelbrock have a selection of power valves so they can be tuned.