The CMC3 (+12V phantom) is identical inside save for an extra bridge across a resistor on the rear. I'm told the CMC6 has superior noise rejection compared with the CMC3/4/5 units, so some of that more complex circuitry must be responsible for that as well.
Testing side-by-side between the CMC5 and CMC6, I can't hear any difference at all just speaking, singing, whistling, etc into them. YMMV.
- Jason