well, i didn't a/b them direclty , so i can't tell you a detailed difference but in general i like the mk2 better, they sound fuller with harder hitting bass more fun and musicality, the mk3 takes a more balanced approach with decent but less bass.
will is offering now mk32 which he thinks is the best right now, but i still haven't tried that.
you can get mmcx of any of them mk2,mk3,mk32 they should sound the same, but the mk2 and mk3 i had where from rholupat with vjc cable, i found the mmcx version with my cables sounding cleaner, so if you have some cables lying around, you can try mmcx
also if you have a balanced source, mmcx would be your better choice to use with balanced cables.