Chesseroo described it in fancier words than I can, but it's pretty simple. Watching a movie, the L-R mains essentially produce effects and flourishes of soundtrack during dramatic scenes. You won't notice qualities of a speaker like midrange transparency or bass tightness watching Star Wars...as long as you hear the effects clearly enough, it would be quite hard to notice deficiencies in a speaker watching a movie. Speakers with similar components (i.e. from the same company) will probably sound more quantitatively different (ex. output, bass extension) than qualititatively different. Beyond output and a little more bass, I can't believe there would be a noticeable difference between the M40 and M60 such that you would feel something is "missing" when watching a movie. As for the Amadeus thing, remember that DD and DTS soundtracks tend to put vocals in the center channel anyway, which is the second point to the comment.
Music is the major thing to base your decision on...if the M60s are more in keeping with your tastes, room size and budget, by all means wait on the sub.
