I don't see how one can be biased if one owns both cars. The M3 is DEFINITELY more stable at speed. I confirmed that again today. Got my 335 up to speed governor around 155mph, then I had to slow down & turn to the left. The back felt a bit uneasy with the manouvre. The M3 takes this in its stride & is rock solid at any speed.
But the the 335 does have better brakes. There's no doubt about that.