The 109.20630 is an AA 6x12. It was sold by Sears but it wasn't made by Atlas. If you want a 6" lathe, your best choices are either Atlas 618 or Craftsman 101.21400 (same machine, different badges).
I can't really recommend the Atlas 9" unless you are starting a collection. They were made from 1932 up until about 1937. The repair parts situation is poor. Between the SB 9" and the Atlas 10", I would take the Atlas if it is a 10F. The 10F has Timken spindle bearings instead of split sleeve bearings, V-belts versus flat belts, and the repair parts situation is far better. Atlas made nearly 100,000 10" lathes, the majority being 10F's. The SB 9" is in my opinion almost always over priced. Beyond 10" the choices increase.