I have the 6600 & absolutely love it (won it in a raffle recently !!) I have used the walking foot & am really impressed how it just swings down & back up - so this is really great.
I have never used the 6500 so cannot comment on this machine.
My other machine is a Bernina (also still really like this, bought it years ago when my children were little & its fab, just not a quilting able machine) had a trickier walking foot attachment so this 6600 is a vast improvement.