I drove 2 different C63s (one without the sales guy), 1 M3 Coupe with dual clutch tranny, and then two different B8 S4s (one manual, drive select & sport diff., and one S-tronic, std. suspension in the dry and in the wet.
These were my impressions:
C63 had a great engine note and impressive accel. Otherwise, it felt very plain. It did not feel at all like a high performance sports sedan, accept when you start it, or get on the throttle hard. Interior was quite cheap feeling (except for the seats). Felt like an 'American hot rod' sedan. The auto tranny sucked imo, with slow upshifts. LSD mandatory!!
M3 with dual clutch definitely had a more sporting feel than the C63 (i.e. better turn-in & better body control, etc). Accel felt nearly as impressive and more linear in delivery (at higher RPMs). The tranny sucked though! Lots of clutch slip at low speeds, like a really bad driver! I couldn't live with this; it would drive me nuts, never mind the expected clutch life. There were also delayed downshifts and unremarkable upshifts (fast, but no engine retardation like with VAG DSGs), and I definitely prefer column mounted paddles that you can always find, with non-racing racks (i.e. not high ratio). Sadly, you really can't hear the engine note - what a shame with such a great engine. Both the C63 (only comes with sunroof) and M3 (no sunroof), didn't have enough head room for me (6'3").
S4 had a better (i.e. more sporty) looking interior than either of the above,