Couple notes as a dealer of both brands - the M21 "Network Card" can be used as a streamer the same way as the PBD Stream X module can. StreamX is a $3K option on the MPD-6. Both can leverage MConnect and JPlay to work as a streamer or serve as Roon or UPnP endpoints. The person above asking if they needed something else if they have a Roon Nucleus on network, no. This will serve as a Roon endpoint and nothing else would be required.
The MPS-X with a separate server is superior to the StreamX module but StreamX is a very good place to start.
Both units are very well manufactured. Bricasti is built like a tank, no question but when you pick up a PBD DAC it is obvious how well it is made. Both made in the US, just on opposite coasts.
The major difference is going to be in the image and a few features. The MPD-6 and M21 tonally are quite similar but the image is different. The MPD-6 is a smidge more detailed with a large soundstage that is tall and wide and has excellent depth and good stability. The Bricasti delta sigma DAC probably has a larger image in terms of height and width but I don't believe it is as detailed and I don't think has the absolute depth of the PBD. The R2R is nice and a different flavor altogether.
Both are upgradable. The firmware on both can be refreshed introducing new features. The modular nature of the Bricasti also makes longer term hardware upgrades possible. Both have amazing customer service and relatively few issues.
Bricasti's Amps and Preamps are absolutely superb and obviously, the M21 will match aesthetically. The M21 has a bunch of filters to fine-tune the sound. And obviously it has R2R, 1Bit DSD and the Delta Sigma DAC meaning you have more options. Playback has no filters. You like it as is or you do not. Fortunately, Andreas knows what most people like and the MPD-6 is very appealing to many.