I have the 7700 and like it a lot, however there are some improvements make in the newer (blue front) 8900 that I believe have also made it into the 9400. I have some of those on my Elna 860 and really like them (quickset bobbin, touch-screen interface, dedicated needle plates instead of the APC one on the 7700, 1/4" piecing stitch).
The things I don't like about the 7700 are the APC needle plate, which is a bit of a weak link with the spring mechanism, and unless you have the AcuFeed feet on, it really has a difficult time going over thicker seams. I'm finding this with Leah Day's current BOM - I was piecing using the 1/4 clearview foot and it was giving me fits trying to go over multiple seams.
I'd have to try the 8900 and 9400 to see if they have the seam bulk issue, if not that would be a strong vote in favor of the newer model.
It would really depend on the price difference, but if I could swing it I would be much more inclined to purchase the 9400.
Michelle