Hehe, I thought of more things!
Shoving a big quilt through a small harp on a machine is no easy task either, so it's very physically demanding, but at least you're sitting while you're doing it. I've read that some people get a saddle stool or something so they can sit while they longarm, but then you have to constantly move the stool too - I imagine they make rollers you can put on them? No idea how that would feel. Apparently loading the quilt can also be challenging the first few times (or more?) that you do it but I know that experienced longarmers can load very quickly.
On a sit down machine, like a Tiara or a Sweet Sixteen, you still have to baste. I believe you cannot glue baste with these machines? Not sure on that, so hopefully someone can weigh in on that. You still have to move the quilt though, and some have built the suspension system that hangs from the ceiling so the quilt is supported and is easier to move. Not necessary, but it's another idea to consider if you don't like the weight of the quilt as you move it.
You cannot use pantographs on a sit down like you can with a longarm on a frame. I mean, you can, but you have to somehow transfer the marking (needle punch then transfer like a stencil), or copy it a ton of times, attach, and stitch through it, then remove the paper later. So, not like on a longarm
Ruler work is different, since you have to hold down the ruler while holding the quilt while pushing it through the machine! On a longarm, I believe you hold down the ruler with one hand and move the head of the machine with the other.
A sit down is purely a free motion quilting machine. It doesn't have feed dogs. There are sewing machines that have 11-12" harps that can also do embroidery and regular sewing but they also cost a pretty penny. If you want a machine that does "more", that could be a consideration for you since it could replace your current machine and then it's not just a unitasker (only does one task) taking up space. Space is at a premium for us right now so I'm not willing to give up the space for a sit down either.
You'll want to consider how many quilts you're thinking about making, and how often you would use the machine, and weigh that against cost, space, enjoyment, and anything else you can think of. Have you tried any of the machines yet? Some people love/hate certain ones. Everyone's brain works differently - some do better moving the pencil, and some do better moving the paper. Stitch regulator or not? How steady is your foot and steady your hands? Different brands have different regulators and some work better than others (both sit down and standing). Some quilters hate basting, so while you don't have to baste with a longarm, you do have to load the quilt and that takes time too. I can glue baste a quilt in well under an hour, but then I have to either wait for it to dry or iron it, and I have to move all the toys to make enough space to do it
