As a V8 5speed owner I'll add my two cents. I've driven both, and I
would guess the S4 to be faster off the line. The 3.6L in the 5 speed is
a little light on the low-end torque--a problem they remedied in the
4.2L. Unfortunately, only in Europe can you find the combination of
5 speed (or six speed) & 4.2L V8.
One of the reasons for the V8 5 speed's remarkable 0-60 time (in the
sixes) is that it can get to 60 with only one shift. Second gear will
take you (just barely) to 60. And one nice feature of that engine
is that it has lots of power the whole way to redline (~7000 RPM).
First gear is also very low, which serves to get the 4000+ lb. into
motion very quickly.
Now, if only I could get it twin-turboed...
Applied Research Laboratory
Penn State University
State College, PA