I have the 8200 which is the same machine but with only 150 fancy stitches. Didn't get the 8900 because I wouldn't use the extra stitches. That being said it is a great machine. It does free motion like a dream and there are 3 different type of free motion feet that come standard. It has a beautiful straight stitch. I don't see any advantage to the 9mm but again I don't do much with the decorative stitches.
The only negative thing for me is the knee controller. It is too far to the side so I put a pool noodle on it to help but I can't seem to get the hang of it but will keep trying.
I use large cones so did purchase the unit that fits on the machine to hold them. Works Great.
I don't think you will be disappointed.