It’s nice to know that the 6600 still holds it’s value. When I purchased the 8900 the plan was to sell the 6600 but everyone (including QB members) talked me out of it. It is a workhorse and I’m glad I kept it, I still use it more than my 8900.
I also think Pfaff is a great machine, I have an older model at my summer home which I use for piecing. Good luck with your decision. I’m sure test driving them will help you choose.