I just got the 6600 last September and have been more than pleased! It was a big jump from what I had been using.
Your best thing to do, would be to go to the Janome sight and compare the specs. From what I can remember, the lighting is no better on the 7700. Yes, you get more throat space. However, I seem to recall there was something on the 6600 that was NOT on the 7700, which was part of the reason why I did not take the bigger jump to the 7700. Also, I recall the dealer saying he much preferred the 6600 to the 7700, and I forget what the reason was ... though I think it was that other thing I can't remember!!
Since then, there has been another model introduced, so if you're switching, take a look at all! It all comes down to, do you need and will you use, the additional features that come with a different machine?