Personally, I have heard both the M3's and the B & W 303's, and I prefer the M3's. They have a more open and neutral that I prefer. The 303's seemed a little 'filtered'. Sometimes voices out of it didn't sound 'human' or natural. The M3's, I believe, also have a little bit more bass, but I am not too sure. That could have been due to placement of the speakers. I could not compare them side by side, thus the different listening positions and environment could have affected the bass performance. Either way, I believe that either of these speakers should be matched with a subwoofer.
