OK' please note that the liking for the 9mm is subjective. If there was one clear winner that everyone agreed on there would only be one feed dog width.
My dislikes of the 9mm
1. I had a very hard time peicing quilt blocks as 1/4 inch has very little feed dogs over the fabric on the right
2. Much bigger issue as I sew a lot of costumes. I had large problems getting the fabric to turn tight curves as the feed dogs griped the fabric a little too well.
3. I also founding hemming to work better on the 5mm.
What I liked about 9mm
1. Decorative stitches
Others I know don't have the same problems with the 9mm. I would try out the 9mm for yourself with projects you do both quilting and clothing.