With my Pi (running Roon) based SPDIF streamer, I definitely preferred the Mystique over the M3 in my system. Adding the M5 to the Mystique using the same SPDIF cable (Shunyata Anaconda ZTRON) brought it to a whole another level; I'm now kicking myself for not upgrading streamers years ago. The V3/M5 combo (assuming you can find a used Mystique V3) actually cost me less than the M3 with the integrated streamer would have.
That being said, I was told by Bricasti and forum members that the M3 sounds its best with the integrated streamer, so assuming the M5 bump is the same for both DACs, plus add in the extra for the integrated solution, and yea I would now like to have that M3 in my system to compare. If it is simply a case of magnifying the strengths of each DAC, I likely would still prefer the V3, but I can't say for sure without hearing both combos.
I'm truly shocked by the difference adding the M5 made. Being able to try the USB input on the V3 vs SPDIF is a nice bonus too. I also have an Anaconda AES cable and actually went to hook it up when I got the M5 and realized the V3 doesn't have an AES input :) Maybe someday I'll get an EVO which looks to have AES... I have an etherREGEN coming tomorrow to feed the M5, and am able to try it both with fiber and copper so that should be an interesting test. I'm now a strong believer of handling your digital signal with the utmost care :)