If you know anything about acceleration, you know that for that m4 to catch up and create a gap between the rs5 that quickly after being so far behind due to a bad launch it has to be significantly faster than the rs5. If they both hit it on a 30 roll the m4 would be gone, which is more likely what would happen in the real world. But again we all enjoy different things if you like launching then get the rs5, but know that after 40 it won't pull as hard as an m4.
I'm not big into street racing but power and acceleration matters to me so to think that a 6 year old F8x can still destroy Audi's newest and best is mind boggling. Again, the rs5 makes the better daily but the Dct m4 definitely feels significantly faster in the car than an rs5. But if 0-30 matters to you more then yeah get the rs5. I think an M4 with comp pack and carbon m performance parts especially in an individual car is way more of a neck breaker than an rs5, but stock for stock the rs5 is sexier.