If you do a lot of quilts that are 9' wide, I would get the 12' frame. You are going to loose a bit on the edges due to the machine carriage. It is also good to have extra room on the side to move the quilting head clear of the quilt to change bobbins, do a bit of cleaning etc. with a quilt on the frame. You do need a lot of room, however. If you are planning on doing pantos from the back, you will need 3' in front and in back and 3' on one side. Frames are usually between 3 & 4 feet wide, so you are looking at 9 X 15 to comfortably fit a 12' frame.
you can store a LOT of stuff under the frame!!!