You want to compare M3 vs 135i. I have them both & reckon the 135i Coupe is the best value for money car around. With JB3 it outruns M3, stops great & handles well.
M3 is weekend car, 135i work car. Interseting what you say about the turbo motor, I haven't seen my 135i run out off puff until 7000 rpm, it pulls like a freight train from low revs all the way through, torque is much better, when it changes gear it just keeps hauling. You might find the sound of the M3 reving confuses the mind sometimes. It's much easier to lose your licence in the 135i. I see no difference in build quality between the two of these, nor with my wifes 125i conv.
M3 has great kudos though & makes one feel good to drive, sounds great at 8000+ rpm can't beat that. You can be lazy in the M3, then hit the "Mad Button" to turn it in to a Jeckyl & Hyde car.