this argument just keeps going in circles. we've got a couple guys stroking off Porsche while the rest of us take a more practical approach. if you'll indulge me, here is my opinion:
the reality is that these cars shouldn't really be compared, because the Porsche cars (Cayman / Boxster / 911 variants) are true sports cars, whereas the M3 is a very sporty luxury car (call it a "grand tourer" if you like, I'm just so sick of how people use that word these days). the M3 sacrifices certain things to be more livable on a daily basis, whereas the Porsche sacrifices certain practical things to be a more performance-oriented car (i.e. a sports car). I once heard someone say that a 911 is a practical sports car, whereas an M3 is practically a sports car.
there are some things about the 911 that you can't get in an M3: steering feel, a lighter weight chassis, etc.. however, the Porsche cars can't come close to the M3 in terms of space, and their interior quality and amenities are not nearly as good (please don't try to argue with me here, I've owned both and there's no comparison - although I loved the look of the 997's interior, there are far too many hard plastics and it rattled much more than my M3 does). furthermore, the ride comfort of the M3 makes it much more suitable for daily driving or extended road trips.
like Dave, I traded in my 997 for my M3, and having owned both I come out right where he does. I have zero regret about making the switch - in fact, I couldn't be happier. I find the M3 to be even more entertaining to drive because its performance is much more approachable than the 911's. the M3 gives you a chance to correct your mistakes in the corners, whereas the 911 just tries to punish you for them. I'll admit that a highly experienced driver will probably be faster in a 911, but I would bet money that 95% of 911 owners would actually be faster in an M3 than a 997 Carrera(S) or Cayman S. the people on this forum that are always rooting for Porsche never give credit to how easy and forgiving the M3 is to drive, but I find it to be its greatest virtue. and I don't think I'm alone in that regard - take a look at the countless comparison tests of the M3 from automotive journalists. the M3 has won more comparisons and received more universal praise than any car I've ever read about.
with respect to the N'ring times you quote, I agree that 8 seconds is a big difference. however, I don't know enough about how those laps are done to put too much stock in them. is there traffic on those laps? were the conditions (ambient temp, track temp, humidity, etc.) the same? were the drivers of similar (if not equal) talent? I read a C&D comparison a while back where an M3 actually beat a 997.2S on a track. doesn't really mean much to me though - I'm willing to accept the fact that in the hands of a skilled driver, the 997 is faster. unfortunately, I'm not that skilled yet, apparently, as I am faster on a track with my M3 than I was with my 997.
Wow, this turned out to be a longer post than I had hoped. but to conclude, I’d say that Porsche’s cars do handle better because they’re true sports cars that don’t have to be practical DD cars like the M3, and furthermore that’s not to say that everyone would prefer its performance to an M3. different cars for different purposes – God bless variety.