The 306 and 510 are two completely different attys, but they are both interchangeable, that is you can use 510 batteries on 306's and visa versa. Think of the 306 as a super mini, and it has a more exposed bridge which makes it easier for some to drip with.
The Yeti/Dura/Titan are all 510s. I am not sure if the Dura-Janty named one has a seperate switch like the kissbox.
You can get the 510s with manual or auto switched batteries. The manuals are most popular.
I don't think the 901s are interchangeable with any of these joye line. At least none of mine do, but I haven't bought a new 901 in a long time. I have used 901 carts on a 510 and 510 carts on a 901 as they will work with each other. I prefer the 510 carts as they hold more liquid and are round like a cigarette, versus the flat tip like a penstyle or 901. You might ask Jason these questions as he sells true 901s, 510s and 306s.