i think this is a difficult comparison because the cars have very different caracteristics.
the m3 is more a sportscar and has attitudes like a sportscar.
the s4 on the other side is a sporty limusine, not a sportscar. indeed it has a powerfull engine but it is not so much fun to drive this car on a racetrack, the m3 does.
the s4 is not build for racing. i know because i've been driven my (old) s4 a few times on a racetrack and also the new one. even with modifications on the car it is not optimal. i had the chance to compare and drive m3 and s4 on a racetrack. in my opinion the m3 has the better properties on a racetrack.
but i guess in everyday use is the s4 better because it has more space, 4 doors, bigger trunk and so one.
i think the m3 and s4 drivers have different requirements to their cars, so that's why i think it's difficult to compare these cars.
btw:
for me are this figures very interesting:
Elasticity (4./5./6.)
Speed BMW M3 Audi S4
80 - 100 Km/h 2,6 s / 3,4 s / 4,8 s 2,5 s / 3,4 s / 4,6 s
80 - 120 Km/h 5,2 s / 6,6 s / 9,5 s 5 s / 6,2 s / 8,5 s
80 - 160 Km/h 10,9 s / 13,9 s / 18,9 s 10,7 s / 13,5 s / 17,1 s
80 - 180 Km/h 14,1 s / 17,4 s / 25 s 14,4 s / 17,1 s / 22,3 s
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