I have not heard either but researched both, the PDB is the most upgradeable sonically. It is FPGA based design done by the guy who invented the FPGA DAC design.
The Nagra DAC that I think sells for $30k-60k is using the low-end code that Andreas Koch (sp?) the PBD designer sells to third parties. He saves the good stuff for his own DACs.
Ted Smith of PS Audio learned how to create his FPGA DAC from Andreas.
I owned a SONY SCD-1 SACD player (modded) and Andreas was involved with that project. Just from that past experience alone made me buy the PBD Streamer-IF streamer with Plink and ROON READY capability. PBD's prior gen streamer
I am currently using the SPDIF of that streamer with my Benchmark DAC3B. It is a killer streamer. As good as the Lunim X1 stream and Sonore OpticalRendu fibre optic streamers. The Streamer-IF is inputting Ethernet, which I find the worst (fibre the best), but Andreas and his partner have made some magic happen in that streamer. If he can do that with crappy Ethernet I expect greatness from his DACs.
My plan is to get a PDB Dream DAC to hook up to the Plink fibre of the Streamer-IF. Use ROON of course.
I am moving my DAC purchase to 2nd in priority since lower priced DACs are rather good these days. My top priority is to get an amp as good as my CODA #16 for my Livingroom. The new CODA SYSTEM 150 (to be released next month) is what I am targeting to pair with the PBD Dream or Edilwise DACs. The 150 amp is supposed to be a little less warm than the CODA #16 (TBD) and should match nicely with the slightly warm PDB DACs. I am using the CODA #16 in my office.