Your decision really needs to come down to how important HD Audio via HDMI is and how important the video scaling and conversion options of the Yamaha are. If you are 70% stereo usage, than I would say go with the NAD.
The opinions you are seeking are a little difficult to give, if the people giving them don't have B&W speakers. Electronics sound different on different types of speakers. I have B&W's, see my signature below, and I had a Yamaha 3900. Overall the unit is good. It has lots of bells and whistles, although the video upscaler was so so. My Digital sources from cable that weren't HD were never upgraded to even HD quality, but they were a little better.
Any way, in terms of SQ out of the Yamaha, especially for music and 2 channel stereo, the receiver only sounded it's best when in Pure Direct mode. In other words, unless you bypass the internal circuitry of the unit, it didn't sound its best. Also, it is important to note that on my speakers, which I used the Yamaha to bi-amp, the sound was very much loud and in your face, but lacked some of the refinement and dimension that a higher end receiver or separates would have.
The main reason I returned the Yamaha though, was because it made my speakers sound overly bright. I would experience listening fatigue far too quickly when listening to music at any volume levels.
If it were me, I'd go with the NAD. Even Yamaha has QC issues. Check out the Z7 thread and the video upscaling problems that has taken Yamaha 2 months to fix... Every company has QC issues from time to time with products. It is their willingness to correct them that counts and I think that NAD's service level has been improved over the years.
Good luck with your choice...