I chose the 6600 over the 7700 horizon because of several things. We had a budget of so much and stuck to it. Also there are several different great things on the 7700 I didn't need. The 6600 filled my needs. I love it. I have also heard great reviews about the 7700.
This is important if you are looking at the 7700 Horizon, my dealer told me that they are coming out with a new 7700 Horizon that will have an easier throat plate to replace and some other things that sounded good but can't remember them all. She said she thought they may put the current one to where you can buy it online instead of just through the dealers. Don't know what the new one would be priced at or if they will lower the price on the one from right now.
Janome also makes the 12000 Horizon (top of the line) and I think someone might have confused that with the embroidery with the 7700. It may pay to wait and talk to your dealer and buy when you can see the newone.