Unlike many others I picked the 6600 over the 7700. My tests showed the 7700 would not sew over a multiseamed intersecting seam, on two different machines at two different dealers. Perhaps the feed dogs needed to be adjusted.
The 6600 had no problem sewing over the same seams. I found the 6600 had no vibration at full speed while the 7700 did have a slight vibration. I thought as did my friend who was with me that the straight stitch was a bit better on the 6600 then on the 7700 and sewed effortless and straighter. I liked the direct number punching of the different stitches better, on the 6600. But that said, I loved the Horizon for so many reasons too. It was not an easy decision.
I had read some bad reviews of trouble with th 7700. Then I read great reveiws. All together it seemed the reviews for the 6600 stated less problems. I dont really understand all of this since it seems the 7700 is definitely a sister of the 6600. But all that being said, if I did not also have another machine with a freearm, and many deco stitches, I would buy the 7700 with the small difference that I tested. Only you can make this decision. They are both beautiful machines. If only we could have as many as we wanted it would be so easy.