WDS vs. FOG
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So I've used SCCM, WDS, and FOG in the past. Recently I left a network (where we used SCCM) to move on to where I am now as a sysadmin. My sysadmin co-worker also left, and became the director at a school district. In his environment, there was nothing, and we both agreed that his scale (about around 800-1200 machines) that FOG would work just fine as he isn't reimaging them a lot (maybe 20 or so a month). In my environment, I chose to setup WDS as we're scaling but not at a crazy rate (we have a lot less machines than the school district, but I figured why not). I setup a SCCM proof of concept over a couple days (per management's request), but I chose not to continue with it because our scale is WAY under its capabilities and showed management side by side the differences in overhead. So even though we're already paying and using System Center for other services (SCOM/DPM), it's too much administrative overhead to take SCCM seriously.
TL; DR= Why are you using WDS or FOG over the other, other than shear preference? I've saw both scale pretty darn well up to a couple thousand machines, and both are efficient in multicasting IME.