These two lenses are very different animals.

The M is much lighter, is in a copal 1 shutter, and has a 325mm image circle. The W is heavier (by 2pounds), is in a copal 3 shutter, and has a 420mm image circle. The W take 95mm filters, the M takes 52mm filters.

Which one to chose depends on your needs. The W weighs almost as much as my M and camera (toho) combined.