I don't have either but I do have the 7700 which is very similar to the 6600. Both have lots of bells and whistles.
My understanding is that the 1600 is a straight stitch only machine..no zigzag, no buttonholes, no fancy stitches, nothing but straight stitch only. Apparently, it's very good if that's all you want. I do have a hankering for one to put on a quilting frame...if I had a quilting frame...lol.