Hi
The offer you mentioned seems to be tempting.
Speaking of design, you will definitely have a better experience while using the phone, and the leather style back makes your phone look more elegant.
Speaking of performance, since your are not a geek, you may not feel a significant difference.
Speaking of software features, there are some cool features like Air Command and Multi Window, which I am not sure if it finds its way to Galaxy Note 2.
Speaking of Camera, Galaxy Note 3 has a 13MP rear camera, and some features like Dual Shot
Speaking of Android update, Galaxy Note 3 comes with Jelly Bean 4.3 and there is no reason not to get updated to KitKat 4.4. But Galaxy Note 2 is still waiting to be updated to 4.3