Do you get "wow'ed" by "massive torque" and don't care about the relationship it has to power? Then go with the turbocharged M3. If you are particular about making power at high rpm's and would rather preserve the throttle response and sound, stick with what you have a supercharge.
I would expect, to some extent, that with a new generation 3'er that last-gen S/C kits will come down in price. Then again this may not be accurate as the next car is turbo. So, it's tough to say. My 2c is S/C today, VT2-575.