Drove my M3 and a 991S back-to-back today
This should come as no surprise, but overall, the newest 911S is just a superior performance car to the M3. It is faster, turns sharper and has gobs more grip.
That said, there were two areas where the M3 was clearly superior IMO.
1. The ride/handling balance in the M3 is better. Sure the Porsche corners better, but you definitely give up some ride comfort for this. It is not harsh, but lacks the suppleness of the M3 suspension. To drive on public roads and especially on longer trips, the M3 is clearly better.
2. The steering (and this surprised me) is better in the M3. Really must say that I hate the electric steering in the Porsche. It really does not communicate like the M3's. it is not horrible, but for a car that is otherwise so superb in its capabilities, a good bit of the tactile feedback of what your car is doing seems gone.
Other than that, I gotta say that the 991S is a phenomenal machine. The engine, while not as smooth revving, feels very gutsy. The exhaust sound is decent. No understeer whatsoever. The car goes exactly where you put it, and even in RWD guise, the limits of traction are very high.
In the end, one concludes that the M3 is a maximally tuned practical transport vehicle, while the 911 is a pure performance machine with creature comforts added in.
Does it blow the M3 away? No. But it is clearly a superior performance car.
Audi S6 * Audi S3 * Porsche Cayman GTS
--Former BMW M3 owner