Both of these digital camps have realized that the "old school" delta-sigma DAC process (of which your NAD DAC is one) is compromised or at least not completely natural sounding to most audiophiles.
Not all designers believe this. The real problem is digital filtering, not Delta-Sigma versus R2R IMO. There are advantages and disadvantages of both technologies.
5 year old DACs are indeed pretty good. However those with the latest 9018 ESS Sabre chips are more technically advanced and measure better. The slight improvement may or may not be audible.
I would be leery of choosing a DAC just because the design uses a particular D/A chip. I believe virtually any chip designed in the last 10 years can be made to sound stellar, assuming that the on-die power delivery is not screwed-up and the implementation of the DAC design is good. One exception I have found is the switched-capacitor D/A chips from AKM. I cannot make them sound good. I tried.
Steve N.
Empirical Audio