PS Audio PWD II vs. NAD M51

In the used market these are similarly priced. Does anyone have an opinion on which is the better DAC?
I'm probably getting one of these two. A little help please?
No experience with the NAD, but I got the PWD II once it went on clearance with the advent of their DSD model.

I am still giddy in love for it even after 9 months.
Hello. I don't have much input, but I'm interested as well.

What will be your source? If computer, there have been several reports by users on another forum highlighting the positive impact of using a good USB-to-SPDIF converter to feed the M51 vs. using the included USB input. The two converters I recall mentioned were Audiophilleo and Yellowpuc.

Don't know if those converters have a significant impact on the PWD.
Get the PWD.. when you want to upgrade there's a simple cost effective path.
I have an Audiophilleo 2 and plan on using the spdif inputs on either unit.
I wish I could help the OP with an informative response but I am in the exact same boat. (I almost made the exact same post.) I am looking to upgrade from a Rega DAC and am considering both of these DACS plus the Chord Hugo.

There seem to be great values available right now for between $1k and $2k new or used.
Can't speak for the PS but my M51 has not only replaced my previous dac with great sound, but my highly regarded preamp as well. It is a very transparent arrangement. If you think you might want to go down the all-digital road the M51 is an amazingly good way to get there.
I have a PS Audio DS DAC on the way. Up till now my NAD M51 has easily beat the EMM CDSA SE (an $11,500 dud), the AMR DP-77 (complete with tube-rolling failures), the Antelope Zodiac Gold, and the Berkeley 1, among others.
The DS DAC just retired my beloved NAD M51.
I have both. They both sound great and would be happy with either one, but if I had to choose keeping one, I prefer the PS Audio DACII over the NADM51. The DACII has a bit more air, warmth and musicality. 
The M51 is a great DAC,   I love mine , but I think they are about to retire it soon.  There have been no firmware updates in quite a while.  I have no plans to replace it anytime soon it's that good.  Good deals on used ones.
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.