I went from a fairly basic 30 year old Janome to the Horizon so can't comment on the 6600. But, I do love the Horizon and would buy it again in a heartbeat. I don't mind the joggle dial at all, although I've not had the other option where you feed in the stitch number. It's great to have the features on it for if and when I need them. Needless to say, it will probably be my last machine so to me it was worth it.
I'm sure you will enjoy yours too