Ok, I've had all the machines you mention. I traded in my 6600 for the Horizon. The 6600 doesn't have a freearm, very annoying. The Acufeed foot is awesome, the 6500 doesn't have it. I also have the 11000SE. That was the first machine I picked up a couple of years ago when I started quilting, because I thought I'd never get the hang of FMQ and wanted the embroidery and/or stippling it offers. Quite frankly I never use the 11000SE and it doesn't have the Acufeed. Also you need to get the Customizer software for sizing the embroidery designs because the 11000SE will only downsize or upsize about 20% on its own. You need to be pretty good with a computer to download updates and transfer to the sewing machine.
I use the Horizon exclusively. The extra throat space is awesome.
I thought I'd be all into the embroidery stuff and bought a whole bunch of CD's with designs on them and just found that I didn't really want to put them on anything. I just wanted to sew.
Oh, and any machine I got had to have an automatic thread cutter. It's like living without a garbage disposal. Just have to have it.
Just my two cents worth.