I have the 6700p and the 9400, note not the 9450. They are both awesome machines. The 9400 as well as the 9450 have more features than the 6700p.
The 9400\9450 has auto pivot and end of bobbin sensor. The 6700p doesn’t have either of these features. It does have a knee lift which compensates for the lack of auto pivot. They both have the HP foot and plate which gives you an awesome straight switch that gives very accurate 1/4 inch. They both do fmq beautifully. The 6700p is semi professional, sews very fast if you want and can sew over thick seams with ease. I like the bobbin winding better on the 6700p. If I had to choose I would get the 9450 as it has more features but they are both terrific machines so whatever you choose you won’t go wrong.