Couple of questions for you:
You mention you are buying a frame for quilting (short/mid arm). The machine you are looking at is for that frame?
Will you be moving the machine from the frame to sew or will it be dedicated to the frame? If you plan on using the machine for regular sewing also, it is a real pain to move from the frame to the table. I wouldn't move mine in a heart beat! There is a lot to set up for the frame quilting.
Why not buy a dedicated straight stitch machine for the frame and the 6600 for regular sewing?
Here is a photo of my set up: