As a diehard Bernina girl, here's my 2 cents... buy the 820 and don't look back, you'll love this machine! Then, at the same dealer, trade your 6600 in on a Bernina 430. You will have two killer machines, one 9mm, one 5.5mm, and you can share many feet and accessories. Not all, but many.
I think it's much more economical to stick with one brand, and you simply must have a back up machine!