Oddly enough, few if any of the alternatives have a streaming (Ethernet) input. Maybe the Classe Delta Pre at a cool $10k and as yet unavailable.
Of the two you listed, I chose the SHD. As it’s in our TV system, I can’t comment on absolute sound quality, but it sounds very good feeding our Audioengine A5+ powered speakers, whether streaming high-res files or decoding the TV’s output. It has worked reliably, and I’ve found the Dirac RC and miniDSP software quite easy to use. Also, it’s a little cheaper than the NAD. Finally, unlike the NAD, it has Dirac right now -- I dislike buying anything for the features that will be released "soon."
People seem to like the BlueOS used in the NAD. But many of us are happier to have the plain Open Home/UPnP interface of the SHD, which is interoperable with a wide spectrum of other units.
The miniDSP does show a little evidence of having been assembled from components, rather than being engineered from scratch. For example, the miniDSP app won’t control it, apparently because the SHD’s Ethernet port is connected only to the streaming board. One can buy a doodad from miniDSP that receives wireless signals and outputs a control signal to the SHD’s USB port, but that is an extra expense and then takes up the only USB input. But really, that’s a small issue, and the SHD does have a fully functional IR remote control.
So for price and function, I chose the SHD. It sounds good to me. I can’t see why the NAD would sound any better.
Hope that helps.
P.S. I think, but am not sure, that the NAD offers only fixed crossover parameters for subs, while the SHD offers complete control of slope and frequency.