If I recall you also have a Kana v2 right and you liked it better than the FK?
How's the kone stack up against the kana 2?
They are both really good and similarly shaped.
If I had to choose just one, I would get the Kone Pure Optical solely based on number of buttons, look of the mouse, availability, and price.
I use the Kone at home (gaming) and the Kana v2 at work where I really only need the back button.
They both have similar grips and work well for me using a palm grip (small hands). I'm thinking they would both work well for a claw grip for someone with larger hands.
The mouse buttons feel great on both (I believe they both use omron).
The scroll wheels are a bit different with the Kana v2's being smoother and a little softer than the Kone Pure's. I enjoy both scroll wheels, however.
The Roccat software is good, but still didn't allow me to bind Ctrl and Shift to the side buttons (used as modifiers).
The Roccat software does TALK though which is pretty creepy at times, especially when you get an "Achievement" for clicking or scrolling a number of times.
The talking is helpful though when you switch DPI, which you can custom set to pre-set steps in the software.
I don't have much experience with the Steelseries software since I use the Kana v2 on my Unix machine. It does do DPI switching and the side button is bound to back without any software installed.
tl;dr: I prefer the Kone Pure Optical