I have the 6600 and am more than happy. I've sewn many miles ... and gone over many many layers of fabric and multi seams while doing the FWS blocks and never had issues!!! (well, other than operator issues. Different Story!) I know I could never have done the sewing so easily and smoothly with my previous machine.
Perhaps within time, I'll wish I had the 7700 or Horizon with the wider bed, though for now, I've done very little quilting, and no large quilts.
Compare the specs of each machine and see what they offer ... and what you want/need! Is there room for your own skills growth and future needs?
I forget now, though I know there is something that the 6600 has that the 7700 does not ... yes 7700 is the higher end machine, but the dealer pointed out one thing as not being there.