The 7700 also has better/more lights than the 6600 (5 lights -- the 8900 only has 3). The serpentine stitch has greater adjustability (this is one of my favorite stitches). I do not believe the fabric guide came with the 6600.
The new version of the 7700 does not come with the table or the soft case.
The 8200 has a true serpentine and does not have the dial that the 7700 has (it is direct input). The 8900 still has the dial. Both the 8200 and the 8900 have no screws in the throat plate to make changing them easier.