well you are both kind of wrong. you cant really simplify something like that. hormones play a role and how much you eat and how much you exercise do too. however what you eat and when you eat it have a huge amount to do with it too. also your diet during your development (in the womb, during childhood and adolesence) may have the biggest impact.
you do realize that everything you listed make an impact by increasing or decreasing the calories you burn, right? that really is all metabolism is. how much energy you burn, at what rate. your body doesn't just make energy out of nothing