One other thing to keep in mind.... I had a 9x12 hobby lite press when I started, it was fine for most of what I was doing to begin with, but and it's a big BUT...
The 9" measurement ran from front to back, meaning I had to turn all my shirts 90 degrees to press a longer design. With a 15x15 press, you can lay the shirt on there with the neck towards the back and the excess hanging off the front, regardless if your transfer is 6 x 12 or 15 x 15.
Having to turn 100+ shirts and line them up sideways was NO TREAT let me tell you.
