What you should use depends on what your goals are and your age. In simplist terms MStak is more of a mass gainer while STAK is hormone precursers. STAK will help you produce more testosterone. If you're a young bull, you probably don't need it and MStak is for you. When you're an old bull like me (I'm 38) you can run a couple of cycles of STAK to give you an edge. You can take both STAK and MStak together too for insane mass building.
Strength doesn't come from physical capacity. It comes from indomitable will. -Gandhi
You cannot dream yourself into a character. You must hammer and forge yourself into one. -Thoreau