I just bought one and the choices were 15 or 21. I went for the 21. I want this to be a long term purchase and not have to upgrade fully in a couple of years. It really speeds up the process! Even with the learning curve of going from domestic to longarm, I'm on my fourth quilt top in 2 weeks. Get what you can but if it means waiting another month to get the bigger one then I would wait and save up a little more.