The only two kinds I've seen are A's and B's. The letter is referring to only the Atty and cart shape. The A type is a cone shape, and the B type is a cylinder shape. From what I've gathered, the B(cylinder) shaped attys and matching carts will hold a little more liquid and give off more vapor.
I personally only have the A (cone) type so I can't compare the difference as far as vapor production goes. And if your like me and moving up from a 510, the A type carts hold more than enough liquid to get me
through the day. I usually use about 2 carts a day.