C63 is the better road car but the auto box, although one of the best, is not very good on track. The m3 is far more involving on track but the brakes do need an upgrade on the front for tack work.

The c63 would be the quicker road car in 99% of instances imo. The torque is excellent and if you can fine a 503 engined one with the lsd then all the better. The 503 engine shares a lot with the sls. My mate has a 503 estate. One of the last normally aspirated Amg engines.