pluses for audi: AWD, V8, great looks, nice interiors, IMO feels more refined than the bmw, more people and cargo room which means it's the more practical of the two,
minuses for the audi: weight, reliability is questionable, 4 doors (which can be both good and bad), very little road feel so you feel a bit more disconnected from the road thant he bmw - not as sporty or fun to drive.
pluses for the bmw: faster, it's an M3 for cryin out loud haha, the sportier car of the two, just the funner car to drive
minuses for the bmw: 2 doors (can be a good and a bad thing), no AWD but that's not always bad either, if the skyline gtr is every "ricer's" wet dream, then the M3 is the german equivalent.
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haha. anyway, although i drive an audi myself and would love to have an S4, i would choose the M3. how bout the MB C55?? that could stir up some debate in this comparo. hehe