[rruff]

It sounds like you don't have the hubs... so how did you decide on 20/24 and 28? The correct # of spokes will depend on your weight, the type of rim (light weight and stiffness generally don't go together), and your intended application.

 

[rruff]

I'm a light guy (65 kgs) and I don't need heavy wheels with 32 spokes.
I choise very light hubs (cheap simultaneous), and good spokes: CT Revolution or stronger Sapim CX Ray.

If these are racing wheels I'd lean towards a 30mm rim (like the IRD Aero, or Speedcific) with CX-rays. The rim will be heavier, but it will also be stronger... and the aero benefit is more important than the ~60g of extra rim weight. Besides, you can also use fewer spokes... 4-8 less per wheel compared to one of the <400g rims.

Or, since you are wanting to use AC hubs, you might check out the AC 420 wheels... you can get them with CX-rays also.