Hurricane good response. I have also owned a B7 RS4 and the carbon build up and oil consumption was a major issue for me as well. I also suffered a crack in the engine block and lucky I was under warranty and was provided a new engine at about 39k miles.
The engine in the RS4 was fantastic but with so many issues I became extremely paranoid and thus ruined the driving experience you should be getting from that vehicle.
I switched over to a 2010 e90 m3 in 2010..traded in the RS4 and have never looked back. I have had zero issues with the car since I bought brand new. O2 sensors failed the first few hundred miles but after that nothing.
Idrive is far superior than the MMI in the RS4 and the seats in the M are far superior as well for long road trips or at the track.
RS4 is rare a beautiful car and comes with all the right tools for a performance sedan. The shifter that Hurricane mentioned was just incredible and I loved the feel of the steering wheel much better in the RS4.
Can't go wrong with either OP. I told myself I would never buy another Audi but when the TTRS came out I ended up biting the bullet and now this is my daily and the E90 is my nice weather car (I know weird a sedan is the weekend car and the coupe the daily).
I still have issues with the TTRS and as fun of a car and brutally fast (when tuned) I wouldn't miss it as much as I would the M if I had to sell it.