Its funny, I had an M3, chipped and heavily breathed on, lightning fast, but when I raced a stock Boxster at Sebring I saw the light and traded "up". The M3 is in my opinion one of the finest cars made. For the money, it is unbeatable, but I still think the Boxster has the handling edge and if mid corner poise is what floats your boat, then you will be happy to forsake all those ponies under the M3 hood.
By the way, I can attest to the stability of an M3 at 170mph. No hands are needed on the wheel to take a quick photo. Not to be recommended for very long
