Welcome to AudioCircle, Joe!
There is no technical advantage to NAS over USB for music storage, imo. NAS has plenty of features and advantages in its own right, but as a music file server it is unnecessary if the player hardware has other storage options like USB or local HDD.
Some hardware music players like Aurender handle local music file metadata better than network files. Avoiding networking may have some sound quality benefits if you are an extreme audiophile.
USB is reliable enough if you get a good brand. You could backup your music library to NAS just in case the USB chokes.
Speed is not a concern. All USB is in the Gbps range, while ethernet tops out 1Gbps, but I doubt your NAS and router and player are all that fast. Good news is that even the oldest 10Mbps ethernet standard is more than enough for 192kHz 24bit PCM stream. DSD files are probably better stored locally.
All that said, it is certainly possible that your USB input sounds worse than your ethernet path for whatever reasons. So you have to try both and decide with your ears.
