It is true that Porsche steering feel, weight and accuracy is second to none. I know for a fact that the M3 steering is slightly over-boosted. As for brakes, even 911 owners have to modify their brakes to cope with track use. Road and Track (Feb. 2001) road-test notes that the M3 has "fantastic pedal feel." On the other hand, Porsche brake feel is fantastic as well, from what I've heard. Steering goes to 911. Brakes are equal.
On the safety side, the M3 is far better. It has side and head airbags for all 4 passengers and the excellent, independent crash tests are public knowledge unlike the 911. This is not to say that the 911 is an unsafe car. In their brochures, they go out of their way to show how much they crash-test their cars. But no one can say, with 100% certainty, that the 911's crash data is equal to the 3-series. In a crash, I want to be in an M3. Safety goes to M3.
M3 has SMG which is a big advantage on track.
911 has a very slick gearbox that is a pleasure to shift.
Shifting gears on the TRACK goes to M3.
For street use, M3 is better because of four seats and trunk. M3 is low profile compared to 911. Practicality goes to M3.
Value for money, M3 is far better. You get a fully-loaded M3 for 55k. 911 starts in mid-60's and goes up to 85k when you start adding options. Value-for-money goes to M3.
You ask a tough question, Paul. Personally, I don't have much respect for a regular 911 because the M3 offers the same performance with better safety, better features, and day-to-day practicality for a bargain price. The only Porsche models I do respect are the 911 Turbo 6-speed and
the GT 2. They are awesome! The regular 911 does not stand out in its class.
Back to your track question: Which is faster on the track with the same driver? So, we have to look at the cars, stock to stock, without any modifications to anything. The M3 is an SMG so its driver will never make any shifting errors. The 911 has PSM which enables non-pro drivers to have faster lap-times. After looking through some of my old mags., I see that the M3 has better braking from 60-0 and 70-0mph from two different magazines. The 911 exhibits less understeer than the M3 on the track and it has "sharper but still moderate liftoff reaction" because of the rear-weight bias. The M3 understeers more than the 911 and weighs 335 lbs. more.
It's impossible to answer this question with certainty. I'd be grasping at straws. I have presented the facts. My gut instinct is the 911 will get a slightly better lap time because if the driver makes a medium mistake, the PSM will correct for it. In the M3, the DSC has to be off for the car to be effective on the track. Therefore there is no safety-net. If the M3 driver makes a mistake, he has to make all the corrections himself. Suppose electronics are banned from the track, the 911 still has the advantage of better, sharper handling, and better steering. The only downfall for the 911 is its manual gearbox. Providing the driver does not make a shift mistake, I think the 911 will get a better lap time. And since, one cannot pass in the corners during track days, even if the 911 mis-shifts, the M3 driver still cannot pass him because of rules. And on the straights, the cars are equal in power, so the M3 cannot overtake once again. The same applies if the M3 was in front of the 911. The 911 would not be able to pass him despite better handling. Given equal drivers, it all depends on who is in front of whom on the track.
I hope I haven't confused you. All this goes to show how close the cars are to each other. The biggest factor is the person behind the wheel and how confident he/she is with his/her car's capabilities.