In a nutshell you have Race W2 and Econo W2's. Race have flat milled surface for a indpendent pedestal to locate the rocker shafts, the Econo's have a cast in rocker pedestal, similar to stock style rockers.

The race version has offset rocker stands and they require a fair amount of work to set up correctly, but just takes time. Either head will most likely require pushrod clearances, sometimes significant. Both can make good power for the dollar if you buy them right. The race rocker set ups just cost more.