I'd say go Calm. The trick with Counter/Mirror coat is not taking more damage to bounce back (save on a sash lead), it's trying to find Counter/Mirror Coat off in the first place! Getting the write timing by getting the enemy to attack you with the right move to fire back is not so easy. More survivability is definitely good, especially if you are planning to use it in OU.
To be perfectly honest though, Milotic doesn't seem like a good pick for Mirror Coat from a strategy standpoint. The thing is that most bulky water types are pivotal their team-- they may not outright wall, but they can take on hits from a wider variety of enemies than say, Steel Types, and also pose a threat/danger to a wider range of enemies. Because of this value, it's best to take good care of the team's bulky water and try to preserve its health. Trying to set it up to use Mirror Coat (and thus be exposed to revenge killing) seems like a questionable strategy.