As someone else mentioned, just because you have a longarm on a frame doesn't mean you can't sit down while using it. I have a 22" Innova on a 10' frame and love it. I switch back and forth between sitting and standing while using it, depending on what kind of quilting I'm doing. My DSM is in a Koala cabinet that's about 35" high and I use the same chair with wheels whether I'm using the DSM or the longarm. I just roll back and forth in front of the frame as I stitch. The only time the chair really isn't useful is when I'm doing a larger design that goes really quickly back and forth - then it's easier to just stand and move as I quilt. So the choice isn't necessarily just whether you'd rather sit or stand. It's whether you'd prefer to manipulate the fabric or the machine, and whether you want to put the quilt on a frame or baste it. BTW, once you get used to putting your quilt on the frame it doesn't take nearly as long as it does when you're learning.
Good luck,
DebD