I would go for the Janome 6600 over the 6500. The Janome 7700 is a beautiful machine and I guess you could "grow" into it, so to speak. I have had the 6600 for 7-8 years now, and I love it. I have quilted many a king size quilt on her. I am not gentle with my machines, I work them hard. After all the time I've had her, she decided she needed a break in March this year. She had only been serviced once since I got her, and that was just for a good cleaning. In March, I had to take her back in. It turned out to be a fuse had blown, so $100.00 later, she was cleaned, oiled and ready to rock. I know nothing about the Juki machine, but if your budget will cover both, then I would suggest the Janome 6600 and the Juki F600. I did a search on the Juki F600 and it doesn't look that much different that the Janome 6600, so, to me it didn't offer much more.
But, like buying a car, you will get a hundred responses and all of them different. And in the end, it will be your choice. I love my Janome 6600 and really have no desire to upgrade to the Horizon 7700, mainly because of the price.
Test drive both if you can and good luck with your decision.