Everyone has their favorite machine brand, but I am so happy that I switched to Bernina. I have had both the machines you mention (Janome 6600 and 8900). I have also had upper end Pfaff, Viking and a Babylock Crescendo and several Juki's. In my opinion, YES, Bernina is worth the extra money. While the other machines were all good, lots of features and all sew well (I never had any problems with any of them), the Bernina just feels better. It's hard to explain. If you are a Bernina person, you will feel it when you sit down to sew on one.
Just my humble opinion,
Solomae