Hi - I wanted to chime in because I have had both machines. I first had a 6600, which I really liked and then traded up to the Horizon. There were many things I preferred in the 6600 and actually found I liked it better than the 7700, so I sold the 7700 and bought another 6600!
The extra throat space was the only thing I found preferable in the 7700. I preferred the bobbin winding on the 6600 (the 7700 does not have an independent bobbin winder). Also, I didn't like the way the thread spool sat on the Horizon, the only way to use the larger cones was to use the screw on thread cone attachment, which does not fold down like it does on the 6600, so I was unable to put the cover over the machine.
These may sound like small things, but they were irritating to me, especially since I had used both. Also, I felt the 6600 was more "sturdy" feeling when FMQ and not so picky about tension. The 7700 does have the built in straight stitch plate however. I love the 6600 and would not part with it.
Just my two cents...
Solomae