The 306 atty IS perfectly compatible with the 510 battery but 510 carts DO NOT FIT on a 306 atty. I have both models right in front of me. The only reason I got a 306 was due to it's exposed wick for dipping. The 306 atty comes with a cap so you don't really need to
buy any carts for it if you plan on dipping only. When I use cartridges I just use the 510 atty/carts because the 306 carts are small and a pain in the .... to fill.
Hope this helps
*Edit* I don't have any 510 whistle tip carts so I can only say that the 510 round style carts don't fit the 306 atty. I was assuming that the design of both carts styles was the same where it connects to the atty, but this may not be the case. Maybe someone can clarify this point.