I bought the 6600 only to trade it in for the 7700 two weeks later. The reason was the harp space and the price was not that different. (500) I like the fact that the 7700 has more quilting stitches, and satin stitches. I also like the lighting better. I have a Bernina so the free arm was not an issue. I tested buttonholes and the 7700 were better.
The things I like about the 6600 I had more then the Horizon are, one it was slightly quieter. I liked the vertical thread spool holder much more. I liked the bed more, as fabric slides around easier when FM Sewing. I like the push buttons, It is more intitutive to me, and feels more direct to get where you are going faster.
Other then these comparisons the machines are very similar. I do like them both very much. If I were on a budget I would have no hesitation in buying a 6600 for half the 7700.
I do think you can get the 7700 for a lot less (more like 2600) Floor models go for even less.