The 6600 is phased out. You should be able to get a better deal than that. Personnally the 7700 does not seem like a good thing to me. I don't want to pay for the embroidery options I don't want to use. Figure out what you really want.
Whatever you decide, be sure to take the new owners' class. I have been sewing for over 40 years but I learned a lot from my 2 classes. for example, I saw no reason for the start/stop button, but I put the foot pedal aside and only use the button now. My knees and lower back are much happier. My dealer told us about the options available. A couple I thought I would want I did not buy and others I did.
Go online and check the list prices. I think the 6600 lists at 1499.