Mainlydave,
You are correct, the model 105 is just an earlier version of the 150. The 105 has been out of production for about 5 years. There are 2 major differences in the design, the first is the controller. The 105 has a set point cook and hold controller. The 150 has smart controls that do a much better job of controlling the temperature in lite load conditions.
The 2nd big difference is the woodbox design. The model 150 has a much easier woodbox to handle.