Theoretically vacs are better.
The carb only opens the secondary when needed.
This is assuming you have it tuned perfectly.
Mechanical secondarys open when you want them to.
The purpose of the pump shot is to cover up the big air hole you make by opening the secondarys when the engine is not prepared.
You are in effect shoving the fuel/air down the engine's throat if it wants it or not.
If tuned perfectly, this can be a boon for upper rpm power.
Vac tend to get better fuel economy, be more consistent, and make better torque.
Mechs tend to use ALOT of fuel and make more hp.
The general rule of thumb is vacs are happier on dual plane intakes and auto trannys are happier with vacs.
Single planes have less vacuum signal, so they make for spongy vac operation. Larger cams can do the same without good tuning.
Mechs like single planes and manual trannys, as a general rule... tuning can overcome alot though.
Dave
Edit: Do not attempt to make your vac carb mechanical.
You do not have the ability to run a secondary accellerator pump, this will cause a giant bog. If your secondarys open without the pump shot, you have a big vacuum leak and the engine falls on it's face.
I have seen guys make their linkage mechanical and then rave that they can now feel the back barrels kick in. What they are really feeling is the car slow down, then speed up when the secondary main circuit finally starts flowing.