I had the Nagra Tube DAC and also spent a bit of time with a Nagra Classic DAC while the Tube DAC was back at the factory being upgraded. I used it with the Classic PSU which adds another 50% to the cost and 5% to the sound quality. The Classic DAC is not a valve DAC, but a solid state (and therefore cheaper) version of the tube DACs. It also didn't sound nearly as good for that reason (although I generally prefer solid state).
The Nagra DACs are very nice sounding, but over the long term I found them a bit bland and over-priced for what they do. Nagra gear also tends to be a little short on dynamics and excitement. I was fortunately able to sell mine for a reasonable price and after going through a few others, ended up with a Chinese made Matrix X Sabre 3 at about 1/10th the price of the Nagra which to my ears sounds better.
If you really do want a Nagra, I would suggest looking for a used Tube DAC instead of the solid state Classic DAC, but really I would suggest giving both a miss.
I can't comment on the Audio Note, not having heard one, but I have always liked Audio Note gear so that would be my preference of the two if those were the only options.
The best valve DAC I have heard (and owned) is the Aesthetix Pandora, but there are plenty of others out there.