Which is best for seedlings, bark or moss?

[PaphMadMan]

The best choice will depend on the species or hybrid involved, the age of the seedlings, your growing conditions, and the care you can give them - and many other factors. In almost all cases what medium you use isn't all that important as long as you can adapt your watering practices and cultural conditions. Seedlings of almost any type will do better with slightly higher and more even moisture than mature plants. There is never a clear single best choice of medium for all circumstances.