I have the 6600 too, got it a few weeks ago and I love it. I agree about the extra Acufeed feet and also the lower tension bobbin case if you're going to be doing FMQ. Personally I prefer the metal closed foot for FMQ too that you can buy as an extra as it doesn't bounce, there's no arm over the needle screw, you adjust it with it's little wheel so it just skims the fabic. I find my stitches are much nicer and my control much better (I don't even find I have to wear gloves with it)than with the supplied open foot, although that does work fine too. Since I got this machine, my piecing is more accurate and my FMQ has come along in leaps and bounds! Worth every penny and you DON'T want to know what I had to pay for it over here!
I guess if you can wait for the 7700 and find the extra money then it would be worth it but I'm so happy with my 9" throat and all the other features, I don't regret it one bit!