I own both the Horizon 7700 and the 12000. The only thing I don't like on the 12000 is the walking foot is an attachment, after being spoiled rotten with the 7700 drop down walking foot (and being a previous 6600 owner, I really like the built in AcuFeed).
What I like about the 12000, is the 9mm stitch width, the design your own stitch, and the embroidery is lovely and easy to learn to do. You have to ask yourself, how much embroidery will I really do, or will a non embroidery machine fit my needs, and then go with the non embroidery Horizon. For the 12000, if you do other sewing other than quilting you may have to buy additional feet in the 9mm size, as the ones for your other Janome wont fit with those wider feed dogs.