Experimental theory

I just want to get a better understanding in general when dealing with these types of questions. Say you're trying to prove increasing "A" causes an increase in "B". "A" is moderately expressed in WT/natural conditions. Which would better prove my case:
Scenario A: I run an experiment where A is increased. Results show B increased 50%.
Scenario B: I run an experiment where an antagonist of A is increased. Results show B decreased 50%.