You can do SID with a longarm. You use a ruler to help stay in the ditch, although some quilters are good enough (not me) to do it without the ruler. I haven't practiced that much with SID on the longarm, but I figure I'm already about as good at it as I was doing SID on the DSM, meaning not perfect but passable. There are a lot of advantages to having a frame, but there are some advantages to the sit down models too. Innova just introduced a sit down longarm recently with stitch regulator -
http://www.youtube.com/watch?v=wBxT49mFsTQ. If I were looking for a sit-down longarm (I already own an Innova) I would look at it.