Ron,
I'm pretty much a Weber novice as well. What I have been told is that, when properly tuned, Webers deliver a more balanced charge to each cylinder than is possible with a dual-plane or single plane manifold. That plus the short runners apparently provides a big torque boost over the four-barrel set-up.
However, because each of the Weber barrels is directly over the intake port and such a short distance away, engine heat is transferred more easily to the fuel remaining in the carb after shutdown. As it heats and expands, the pressure rises in the system (what the Weber folks apparently refer to as percolation) and the pressure is relieved when fuel finds a way out through the throttle shafts. This doesn't happen with Holleys as easily because of the difference in construction and position further above and away from the intake ports. --Or I could be completely misinformed, but it seems plausible.
