I have the 6600 and love it. As for the 6500 pulling the fabric down into the bobbin area and the 6600 not doing that - I'm guessing that your dealer may not have sewn a whole lot of triangle points on the 6600. It does in fact do this. I'm told that you can purchase another plate that has a smaller hole to prevent this happening, or you can work around it like many of us do by using leaders and enders when piecing. As to having the accufeed system get in the way when sewing bulky things - from what I have heard and seen, it is not a problem. While it looks like it is just hanging there in mid-air, the clearance underneath is quite adequate.
One thing to keep in mind - when I purchased my 6600 (at Christmas), I was told by the dealer that the 6500 was no longer being made. That is, of course, hearsay, but if I were looking at a 6500, I would also be looking for a seriously discounted price.