If you are practicing yoga with a soft workout, you will often experience the action of sitting on the mat, and a little thicker, softer yoga mat will make you more comfortable. But if you're practicing a Yoga, Flow Yoga, or Ashtanga Yoga that's a much more beating type of yoga, then you need a thin, hard mat, and too soft yoga mats are less convenient to move.
Some of the more fastidious people will also feel too thick yoga mat hinder their contact with the ground and so on, in fact, the thickness of the yoga mat or not mainly to see personal preferences. If you do the action is not so static, nor like running as much sweat, is in between the two, which kind of cushion to use? I will answer "or choose a thinner." Thick mat (more than 5 mm) lose the feeling of contact with the ground, do a lot of action will have "distortion" feeling. In foreign countries, most yoga practitioners love to use thin cushions, that's the truth. If you feel that the mat is doing some kneeling action, knee discomfort, it is possible to cushion the towel at the knee.