I've had both and compared them side by side. Aesthetically the EMC-1 is better. The Opus 21 is nice, but more mundane looking. However, in the sound department the Opus 21 is better. It's very close, but my ears preferred the Opus 21: slightly more organic sounding, just a tad more dynamic, and just a bit more dimensional. The Opus 21 is also made locally and I like that. Every now and then the Opus 21 acts funny, but never has serious trouble. The EMC-1 has functioned flawlessly since day 1, except the Back button sticks every now and then in really hot weather. I have not been able to sell my EMC-1 on Audiogon (probably priced too high) so I'm using it as a transport with an external DAC and it serves that purpose extremely well.
Like I said it's close with these players, but I've tended to prefer the Opus 21 over the EMC-1 based on how it sounds. It's cheaper too, new that is.