They are the exact same engine except for the cam. Meaning you won't have trouble finding any parts for it, as its basically an A6. I know this b/c thats the engine going into my car. I gave up on my search to find a low-mileage A6 and decided on the ZC instead.
So if you are buying an engine from an engine importer it would be cheaper to buy a sohc zc than a usdm a6. I would base that guess off of the dohc zc is cheaper than a usdm a6. And if a sohc zc is really just a hot rodded a6 than why not go for it?