This topic has been hammered to death, and will continue to be hammered to death until we all get to drive the V8 M3 and determine for ourselves just how much better it is than the 335i.
If you primarily like your 335i for straight line speed, occational back road runs, and the luxury of the brand, it is the perfect car for you.
If you want a more "raw" machine that not only is fast, but sounds and acts fast, and you love to rev the engine to have it make power the new M3 may be your car.
Spend the next couple of hours using teh
button (above) and you will get to read some great (and very long) arguments about the 335i vs. E92 M3.
Good luck!