the dual plane manifolds are partitioned off between the primary and secondaries(think of it as having 2 carburetors, one flowing into a primary manifold, and the other going into a secondary one), supposedly to increase velocity. i guess the idea is that since the fuel has less space to fill it will move quicker and stay atomized better on the way to the cylinders. maybe somebody else can explain better -- you can see the partion in some of the pics of the dual plane manifolds.

the C-series is a single plan(like factory) manifold. I've got one on my 20R hybrid and i have no complaints. i went from a 20R factory manifold with a 22R carb to the offy with a weber and it noticeably increased performance.