Driven an M3 DCT and a C63 AMG. The Merc has a fantastic engine, sounds epic compared to the BMW and has masses of torque. I didn't like the interior though at all. Other might say there is nothing wrong with it but I didn't like it. Corners like a loony mobile as well. The BMW doesn't sound as amazing as the Merc but that's not to say it sounds shit, just different. Its engine behaves differently in that it loves to rev. Felt a lot more planted than the Merc and found it a better place to be in than the Mercedes. Not driven an RS4 so cannot comment on that one. Out of the 3, it would be a difficult choice but for me I'd err towards an M3.