I am most definitely the opposite of Leah Day. I hate sit down quilting and was never any good at it but my stand up longarming skills are definitely something I am happy with and others are as well. My brain is wired to "move the pencil not the paper" and I also hated the sandwiching basting process with the white hot passion of a super nova! Frame quilting completely eliminates that step and to me moving the machine head over the fabric is much more natural than moving the fabric around under the machine needle. But that is me. There are some amazing and very talented award winning quilters that do sit down work and on a domestic no less. Diane Gaudinski is one and Janet Stone is another (although I do not know what kind of machine Janet Stone uses or even if she uses an extended throat or not. I know Diane uses a Bernina, or at least she used to.)
Because you are already used to frame quilting, I strongly encourage you to try out sit down models at shows and use your current machine to do FMQ on small projects. If you find you are adept at it and enjoy it and are happy with the end result then you can start your hunt for a sitdown. Just about every LA manufacturer makes an extended throat machine for stationary quilting. Additionally HQ offers a set up on a smaller frame but I don't know all the details about that. It may be something to consider.