The 6600 has a start stop button so you can sew without a foot pedal.
My 6500 is a dream come true. It is a machine that likes what it likes. It is choosy about thread but not all the time. The thread cutter is wonderful. As is up down button.
For that price you can't go wrong. I have had mine for four years and I would consider it to still work like new despite making maybe 60 or 70 quilts, doll clothes and all sorts of other crafty things. Mine rarely has a chance to cool off.