I have a 7700 and I love it. I can't compare it with the 6600 since I'm not familiar with it. The accufeed system is worth tons. I love the way the 7700 free-motion quilts. It is smooth and easy to control with the speed control feature.I sewed with the Bernina which has the stitch regulator and actually liked the Janome FM better. It is quite heavy to carry. You have to decide what you want a machine to do for you and if the extra money is an issue. I took a class that taught me a ton of neat things for the store where I bought it and would recommend it if it's available.

The 7700 is worth the money for lots of reasons; but minus the embroidery. It is one terrific machine!

Leah Day also has a Juki TL98 QE machine, I watch a video online and I have 2 of these machines. I piece with one and quilt with the other.
If and when these give up the ghost I was thinking about a Janome.