Lighter wheels are virtually always better for performance. Whether a particular change is worth what it costs depends on the car, the price difference, and your budget.
The nice thing about weight savings is they help acceleration, braking, and handling. A pound of un-sprung, rotating mass that is out at the end of the car is worth more in savings than a pound taken out of the passenger compartment. How much more? You'd have to do a lot of calculations to come up with any reliable formula. I've heard people say it's a factor of 4, factor of 10 etc. The reality is it all depends on the vehicle and the situation.
I have accepted a new position at Tire Rack and am no longer in the sales department. firstname.lastname@example.org
is available to assist with M3 questions.