As others before me have done, welcome to MobileRead.
The Cybook Gen3 directly supports mobi format. As others have mentioned format conversion is an option for all readers.
Since the screens are the same for the "big 3" in the US and the price difference is so small as to be meaningless, it comes down to which one feels better in your hands. Which one has the controls where they are easy for you to operate. I have the older Sony and kept it because I find it easier to operate than the new 505. While the 505's screen is better, I did not find it so much better that I had to upgrade. For me the Kindle is not an option as the control buttons are not where I can use them easily without excessive page turns. If I was in the market for an e-ink reader in the US today I would pick the Gen3. (With the Sony 505 as a close second.)
Fictionwise is also a great place for getting books in many formats including Sony's LRF and mobi.
Good luck on your quest. Keep us informed about your decision.