Having looked at a side by side comparison of NDS to NDX2:
I’ve laid out where they compare equally spec wise and where one is superior to the other. Subjectivities as to how they sound depend on the equipment paired with, cables, power and speakers and what you prefer, but there could be a feature which helps with how you will specifically use it that can tip the balance one way or the other.
Digital Out (S/PDIF)
Ethernet 10/100 Support
NDX 2 is superior:
Audio Inputs : More inputs and native support for Chromecast, Airplay, Spotify Connect, APTX HD, Roon, Airplay.
Network Support : WiFi support for 802.11 b/g/n and ac
Multiroom support for streaming
Audio File Support - WAV to 32 bit / 384kHz instead of 24bit / 192kHz and higher frequency support on FLAC, ALAC formats. Also DSD 128 support.
Support for higher quality PS upgrades (Such as 555 PS DR)
Improved Zigbee bi-directional remote control (new gen)
Over the air updates through device
LVDS Low noise network card (next gen NAIM network card)
Colour screen with Album artwork etc.
NDS is superior:
Burr Brown PCM1704 DAC (NDS) versus Burr Brown PCM1792A DAC in NDX2)
NDS also has all the brass sub chassis with steel-sprung floating suspension system in the same way that the ND555 has.