Yes, the Opus will be shipped with the ePub Firmware but there's absolutely no problem installing the mobi firmware from Bookeen's site.
The two devices and their software are more or less the same. The main difference are the buttons. The back- and the menu-button moved from the left to the front side and that's definitely an improvement. The +/- button on the right side to in- or decrease font size are gone. And on the Opus you can turn pages aditionally with two new buttons on the right.
When you love your Gen3 you will love the Opus, too.
