The Horizon is considerably more expensive than the 6600; it wasn't worth the difference to me. My husband wanted me to get the Horizon because he always wants "the best", but it was too big for my cabinet (and I love my cabinet). So I got the 6600 and a Janome 350 embroidery machine--buying both machines cost little more than the Horizon.
The 6600 does not a free-arm, but that wasn't an issue for me because I have a good Kenmore free-arm machine. I have had the 6600 for over a year and love, love, love it!! I totally agree with sewnoma's comments.