here is a link to this same discussion back in 2010. Pretty sure it has some of the info you are looking for. If all else is equal, the larger throat space is a wonderful thing whether you free motion quilt or not. No experience with the 7700, but I do have the 6600 and it has been an awesome workhorse for me. Would not give it up. 10 years and not one issue with it ever.
Janome 6600 Memory Craft vs Horizon 7700 QCP