Here is a comparison chart from Janome.com listing the machines side by side. Seems the number of stitches is the main difference. I have the 6600 and added it as well to compare. Looks like the 6600 came first, then the 6650 and now the 6700 which came out last year I think. I love my 6600 and it has been a workhorse for me since 2008 with no problems. I too do home dec and handbag projects, even repaired a tent one time.
http://www.janome.com/machines/sewing/mc6700p/