People say that the 901 has more flavor but less TH than the 306/510 and, I have never used a 901 but, my guess is that is just because the 901 is vaping at less wattage, because that is what they generally say about less wattage on the 306/510. In other words, if you want more flavor (but settle for less TH) on a 306/510 then just try a higher resistance atty or, try less voltage on your VV battery.
But, again, have never used a 901 so this is just a guess.
As far as the 306 and 510, the 306 has an open bridge while the 510 has a closed bridge. What this means is the tube that covers the atty is part of the mouthpiece/drip tip on the 306 while on the 510 it is part of the atty. This is an advantage for the 306 re cleaning as you can see what is going on with the coil on the 306 while on the 510 you can't.
(The I06 atty from Ikenvape is a 510 sized atty that is open bridged. 510 sized but open bridge like the 306.)
The 306 is cigarette sized like many of the low MAH batteries they sell in cigaratte stores (like BLU, etc), while the 510 is a bit bigger. The atties and other accessories from both work on each other as well as on Ego batteries.
The big advantage of the 510, is that there is more stuff out there for the 510. More types of cartos, clearos and the like... and a LOT more drip tips.
I go with the 306 but others go with the 510 (or 901). Different people prefer different things, so there is no one best choice it is just a matter of what is right for you.