I virtually always take point defense, as it has no weight.
ECM may be a better idea for larger ships that are less affected by the mass, as it's a more reliable way to protect from many missiles/mines at once.
Location of the module is also important. Some ships have bad utility hardpoint locations that make PDTs less useful. You need to know where these hardpoints are, and make sure they have line of sight to any incoming missiles. ECM does not require this.