Well, a 911 is not that expensive to maintain. Especially mine had only 63000km for a 95 air cooled. It's not like a Ferrari. If you do the regular maintenance (oil change, etc) it's about the same price as a bimmer. Price for parts are about the same. Beside their stock aluminium shifter at a retail price of 600$.
Concerning the driving experience, all I can say is the 911 felt more solid than the M3. Everything is hard and feel more solid. Especially the steering feel. It's a charm at high speed. The car is so stable. You also feel everything in that car. Maybe a la e30 M3. These days, we lost that with the e46 m3 and the new 911 (996).
Performance wise, the M3 has more low end torque than the 911. My 911 has nothing until 4000rpm. Maybe a chip could solve that. Also US M3 has more low end torque than the euro M3, but the euro one just MOVES at high RPM.
However, for 650$ less each month to own an euro M3 instead of a 911, I chose to sell it. If anyone has any question regarding bimmers or Porsche, let me know. It's my 4th bimmer in 2 years and I did my homework each time before modifying each of them! (I owned a '01 e46 325ci, '98 e36 M3 and'00 M coupe).
Ed