Yep the RS4 is a prestige car but I'd at least feel a little more comfortable if it were a CPO (which there are several of them around). With a decent warranty backing it I think it'd be less risky regarding it possibly being run hard.
While Im not a huge fan of the CLK class Mercedes I had contemplated the C63 AMG but unfortunately they are not competitively priced in regards to the audis. It is a complete monster but when I sat in it at the auto show it would NOT be a great daily driver. Plus I for some reason feel too young to drive a Merc...yeah I know its weird.
Manual tranny is not an issue for me and is somewhat preferred at the moment. I have gone without it for 3.5 years and miss it!