The M51 would have an advantage in two areas.
The first being is digital volume implementation being more advanced in the M51 offering 35bit as compared to 32bit in the PS Audio. This allows the NAD to attenuate the signal a bit more before truncation. If you plan to hook up the unit direct to the amp, the M51 has an advantage, especially when it comes to proper high resolution above redbook.
The second is the HDMI interface which gives you an additional option that few other DAC's offer.
The PS Audio also has some tricks up its sleeve as well.
The Digital Lens, which is now called NativeX, and the numerous filtering options give you a bit more flexibility. For redbook audio, the PS Audio has it by small degree.
The M51 does have it when it comes to high res files with its better noise floor, well sorted out processing, and higher depth digital attenuation. It goes just a little further with detail retrieval while remaining at complete ease. The PS Audio has a slight but less warmth in its sound when direct comparison is made.
