The level of comfort you will require for your fitness mat will depend first and foremost on how frequently you train. The more often you train, the more comfort - i.e., mat thickness - you'll want.
The thickness of your mat will also depend on what kind of activities you need it for.
For example, if you're doing gentle exercises such as stretching or Pilates, during which you will be barefoot and often in contact with the floor, you should opt for a soft mat that is at least 5 mm thick.
It will be more comfortable and protect you from the floor.