Ive got an rs4, its a nice car but as said not the most exciting to drive, loads of grip round the bends, like its on rails in the dry, you never struggle to get power down either. Driven an m3 and they feel a bit 'racier' to drive and just a bit more alive if that makes sense, but a nice one was out of my budget. Never been in a c63 and even more out of budget but heard they go well, sound great but arent great handling wise, also i personally think the general build quality isnt a patch on the rs4 or m3. I guess there all good in their own way so prob best to test drive them all