That depends on what you want from your Kayfun.
Kayfun 3.1 and The Russian are essentially the same thing. They are the "everything and the kitchen sink" models and are the largest because of it. They include a catch
tank below the coil, air flow adjustment screw on the side, one-way valve on the side for easy refilling (Or fill through the top like other clearos), adjustable contact pin and a 510 drip tip adapter.
The Kayfun Lite lacks all of the above. You fill it either from the bottom through a small screw hole or through the top. The Kayfun Lite does use a 510 drip tip so it does not need an adapter if you want to change the drip tip.
The Kayfun Lite Plus is the same as the Russian 91%. These are like the Kayfun Lite, but they have adjustable airflow and the adjustable contact pin. However, the airflow is on the bottom so you have to remove the
tank to adjust it.
The rest of the differences are cosmetic rather than functional.