In my view, the ECB plays neither of these two roles. In fact, the ECB is allowed to play one role only, and that role is clearly defined. The ECB has to safeguard price stability. To be more precise: in the medium term, inflation should be below, but close to, 2%.
I admit that it has been a challenge to live up to that role in recent years. The weak economy and low inflation have obliged the ECB to pursue a very loose monetary policy. This includes the use 1/6  BIS central bankers' speeches  of some unconventional instruments – bond purchases, for instance.