I dive with a BP/W and use a slung bottle on a regular basis even with doubles. Whether you call it a pony, deco, or stage it's still a slung bottle. I don't care for back mounted bottles as I can't see a leak and don't care for another pressure guage clipped off to cause confusion. Practice with a slung bottle will be beneficial if you intend to get into technical diving. If your buying a dedicated cylinder I would get a 40 as the price is about the same as the 30's you'll need the extra capacity as your skill increases, and it will disapear in the water. Make your own stage rigging and save a few bucks. For those of you who travel I have used a rented 80 slung and once you practice with it it will disapear under your arm also. Dive safe.