The M class clockworks are slightly tougher than the C class clocks. They have a small chance of dropping Metalite, which makes them prime hunting for both shadowknights and Paladins.
Like all clockworks they have a high resistance to magic and fire.
The MC clockworks come with a "frost module" which gives them an extra attack of cold. While not as annoying as the force attack, it can make them more difficult to fight.