Playback Designs MPD6 or Bricasti M21?

Bricasti network card is a renderer and not a streamer. This means you need a Roon Core or a UPnP support player like MConnect to be able to stream music. 
I used both Mconnect and Roon with M3 and preferred Roon for both the sound and UI. Ultimately the Bricasti network renderer was not a competition for a dedicated Aurender streamer. However, it’s a nice to have that can hold you over until you get a better streamer or if you’re between streamers and want to continue to listen. It doesn’t sound bad.

Bricasti is Roon ready so just connect Ethernet cable to have it sit on the same network as your roon core and you are good to go for tidal, qobuz and local library files if you have any. You can also use nucleus as a streamer feeding M21 via USB but I doubt that’s going to be as good as just using the M21 as roon end point. 
 

What I was trying to say above is there are dedicated streamers that perform better sonically than the Bricasti network renderer. Do you have to buy a dedicated streamer? No.