Tough decision. I have heard both, not together though, and they are both excellent. I always loved Bricasti a little more than Esoteric until the latter came up with their discrete DAC design for the XD models. Now they are arguably better. Although Bricasti is still amazing.
My impressions were that the M21 was more muscular and rich sounding. Which could be good or bad depending on your system and the balance you are trying to achieve. The Esoteric is pretty neutral, not bright at all but more even handed and easy to slide into most any system with likely success.
If I was buying one right now, the Esoteric would be my choice based on my current system. That could easily change if I swapped out one or two components though.
Honestly at that level they are both great. You really need to try and hear them for yourself to decide. Or just buy one, try it, and flip for minimal or no loss if you don’t love it in the end.
