Different systems, and one person's opinion, so grain of salt warning....
I think it may be more the surrounding pieces, not the DAC itself, that are different. (EDIT for clarity: I mean the DAC CHIP itself, and the inside the box changes: output stage, I/V stage, etc. Didn't mean the other equipment)
To my ears, the new M12 had a slightly more balanced tonal quality. I never seemed to lose details with the M51, but the mids seemed recessed sometimes (that may have contributed to the huge soundstage they seemed to throw), and the highs also seemed slightly rolled. Again, no seeming loss of detail, but that proverbial "air" wasn't fully there.
The M12 seemed to flesh out vocals a bit better, and did seem to have more air up top without any harshness. At the dealer, I was able to plug in my USB stick and play some of my music. I was able to discern a large soundstage still, but a more even/prominent midrange especially for female vocals, piano, etc. And cymbals on good recordings had a nice decay to them that could be a bit muted on the M51.
Just a couple examples. As Purk said, the M51 still is available, so as a less expensive option than the M12 is still a good option. I am just one of those who agrees that it betters the M51 in sound quality.