I didn't feel much difference in the keybeds.
You won't have any trouble navigating. In fact the 900 is easier to get around and saves more stuff automatically.
As far as 76 keys, now that's a different story. Only you know if you'll be happy with 61.
You will have to rework a lot of your Songbook entries if they point to factory styles because the locations have changed and it will call up the wrong styles.
The 900 has quite a few new and better sounds, particularly saxes. Go through the Performances and you'll find some pretty good settings.