For USB, it is a way to connecting peripherial, similar to firewire (IEEE1394). I guess you mean USB vs DLNA should be USB vs LAN (network) as data source, right?
For the case you describe,
If you use USB, the setup is simpliest and cost should be minimal
If you use LAN (NAS -> hub -> DAC), the setup is a bit complicated, but you can upgrade lot of things later, like the network cable, and no need to match USB with NAS (as not all USB DAC is detectable by NAS)