I drove both the 335i and the M3 this past weekend (well, M5s and M6s as well, but they're in another league). On the skidpad, I probably couldn't tell a difference--you just don't have the length of track nor raw acceleration (unless you wanted to be OFF the skidpad) to judge. Handling was good on both, acceleration was strong. The ONLY time I could have felt a difference was if I had been able to put them both on the "long" follow the leader course and then only on the back straight. More important than engine size and horsepower would have been the fact that the 335 was a true auto and the M3 was DCT allowing for MUCH shorter shift times.
Would I buy an M now that I have a Si? Probably, but that's because I have the Si for a daily driver. It's much more well-behaved for traffic.
I took my Si up to the Dragon after the school and I have NONE, ZERO, NADA complaints about her handling, acceleration and general fun factor. Would an M have been faster? No, not really--between the traffic and the Tennessee Highway Patrol, we would have been in a dead tie when the road flattened out.
Drive what you like and like what you drive.