Hi there
I have the 6500 and love using it. I came to it from a 30+ yo mechanical singer, and the features on the 6500 were (and still are) tremendous for me.
Now, with a couple of years of using it, I still love it for piecing, but my removable walking foot never seems to work quite the way I feel it should, and I often use the hopping foot for SITD rather than faff about with the WF. If I had a windfall I would definitely look at the 6600, purely for the built-in walking foot.
My feeling is that if you could stretch to the 6600 you would be happier in the long-run, but if the budget won't sustain the extra, then you would still get a great machine with the 6500.......
Cheers, K
(maybe I should ask Santa for a new WF lol)