The basic power supply is different between TIG/STICK/PLASMA and MIG. Constant current vs constant voltage. While you can build a supply that does both, the cost would go up a lot. Also the complexity of the machine would make it less reliable, and when it does break down, you are out of action completely. There are some tricks to get different processes out of different power supplies, but they never perform as well as the real deal. I do not know for sure, but I suspect that the stick welding output on the MTS is using a trick of some kind to allow the CV supply to work. Another trick that would give you what you want would be to use a PowerPro 256 then add a voltage sensing wire feeder. But again I don't think you would like it. While they do the job, they are fussy to weld with and not the thing for a home user.
Long arc, short arc, heliarc and in-the-dark!