Lots of good comments on the difference between the "sit down" and long arms/frames. I have a Gammill bought used and while I was doing some twin size quilts on my DSM and loved FMQ, have to say that the strain on my shoulders and the difficulty getting a quilt sandwiched (especially when you can't get on your knees!) really sent me in the direction of a long arm. Fortunately, I have space for one in my basement. I love FMQ and using rulers on my long arm and the process sures seems much more intuitive than moving the fabric.
One concern I hear others says is that they can't stand to long arm. I use a drafting stool (I hate pantos so not doing them often is a blessing)while I work with rulers and most FMQ--I stand to do my basting and if quilting a long line across a quilt. So needing to sit is possible.
One other concern is distance to a dealer for technique support and service. It is darn near impossible to take your long arm in for service, but I've found excellent service via the phone and the dealership does send techs periodically. And frankly, my friend that sells sit downs talks about how heavy they are and makes it sound like you don't just drag them in for servicing either--so that seems like a "wash" to me.