Hi James,
Its hard to say if a plant prefers submerged of emersed, but they should grow faster emersed. Apparently some plants should at some point in their life time spend some time out of water - Don't remember where I have read that from.
Many suppliers grow emersed plants in rock wool as it does well in soaking up water for the plants. I think you can substitute if for something like cat litter - foxfish mentioned this in another thread too, I have certainly grew plants successfully in cat litter, I know some will use coconut coir as a growing media for plants too! I can't say which is the best as all the plants I've ever kept emersed came from my aquarium and before I planted it in the tank I would have removed the wool. But you can perhaps experiment with it in your setup and report back in the thread.