Growing in moss is fine during the growing season, as Dens like lots of water while growing. But come winter when they have stopped growing and have finished or not yet started blooming, they like to be really dry, like going a week or more dry. So, I think bark would be better. Medium bark or fine bark is OK, but if you use the fine bark you have to be particularly carfull to use the smallest pot possible, as fine bark will stay wetter longer. Note the Dendrobiums in the center of the picture below and how small the pots are relative to the plants.
