If cost is not an issue then I would go with the Horizon just to have the 11 inch throat.
If the throat size is not important to you then go with the 6600 but make sure you get
the table. Makes a big difference.
Edit: More importantly, have you tried the Horizon and the 6600?
If I were you I would test drive them first. Some love Janome and
some hate it for whatever reason. So it's good to test drive
several machines and see which one you like best. When I went to
the dealer I already had the 6600 in mind but I did test drive it
to see if I really like it. Don't just go with the herd.
Make sure you have good after sales service too.