If looks and performance figures weren't enough to help you decide between these two extremely competitive rivals, then maybe this video will.
With 381 PS and 475 (350 lb-ft) of torque at its disposal, the facelifted Mercedes-AMG A45 is back on top, so to speak, as Audi's RS3 is "only" good for 367 PS, to go with 465 Nm (342 lb-ft).
In a straight line 0-100 km/h (62mph) drag race, the cars remain excruciatingly close though, separated by just 0.1 seconds - in the A45's favor.
Of course, if you really want to see what's what regarding these two cars, you definitely need to take them far beyond 100 km/h (62 mph) and see which one packs more of a punch at Autobahn speeds. Other worthy tests would be of course taking them around a race track (which we don't get to see here), however, also to a lesser extent, trying to figure out which one sounds better when revving its engine.
The video starts off with an exhaust note showdown, which we won't spoil for you. However, as far as we can tell, one car is definitely a decisive winner in this category, which is relevant because both feature optional exhaust systems - 'Sports Exhaust' for the RS3 and 'Performance Exhaust' for the A45.
Concerning straight line acceleration, let's just say that watching the entire video will clarify tons of things regarding these two cars, though make sure to check out the video's description in order to see what impressions the reviewer felt like sharing.