Sailbags (storage) Turtle Bags etc. Waterproof or Breathable?

I need to make a couple sail bags for storage. The bag that came with one of the newer sails (asym from Precision Sails online sails) is coated fabric and too small, all my other bags are breathable. My gut tells me they should all be breathable so I don't get mildew, what say you? Also same question but for a turtle bag. Sails stay onboard for the most part year round.