super effective is 2x, STAB is 1.5x.
Well if your zangoose loves you like heck, rturns base power should be 102, and with stab it will become 153.
With shadowball- it has a base power of 80, 2x on mewtwo makes in a base power of 160.
They're both physical moves, so they do similar damage.
You have to check a few things when thinking about what type of attack to use:
Do not be like the classic noob:
"Well my slaking needs brick break so it could beat other normal types, DUH"
"Look you tart: Return will do MORE damage than brick break on a normal type when used by slaking: 102(1.5)= 153. 75(2)= 150. Don't bother."
Think about things like that when choosing your attack. :-)