Hard to give advice as we don't know enough about you (weight, etc.), the intended use of the wheels (racing, training, daily use, etc.), and your riding style (powerful sprinter, spinner, etc.).
That said, I had a set of Fulcrum Racing Lights (tubular) that were bomb-proof but weighed about 1250 grams, if I recall correctly. I also have a set of Stans 340s built on Extralite hubs (20/24 hole) w/CX-rays that weigh 1120 grams. See http://weightweenies.starbike.com/forum/viewtopic.php?f=3&t=99279
I weigh 156 lbs and am not a sprinter or powerful rider, so the 340s are plenty stiff for me even out of the saddle grinding up 20% slopes. My builder (Troy Watson at Fairwheel Bikes) didn't worry about lateral stiffness; rather he worried about the spokes in the rear loosening as the rims might flex with only 24 spokes. No problems yet. If you have them built w/ 20/28 I think that would yield a strong enough wheel for most uses. Presumably the hub choice will also have an effect on this. I've read reports from other WWies about 20/28 builds that have been fine. You can get a set custom built for about $1000, or less if you use other hubs. Fulcrum Zeros are much heavier (about 1435 grams) and probably cost several $100s more.
But it all depends on your needs and riding characteristics...