M rated helmets have a larger peripheral vision.
http://www.smf.org/helmetfaq#aSAMKdiff
What are the differences between the SA, M and K standards?
The SA standard was designed for competitive auto racing while the M standard was for motorcycling and other motorsports. The K standard was released to accommodate helmets used in karting. There are three major differences between them:
The SA standard requires flammability test while the M and K standards do not.
The SA and K standards allow for a narrower visual field than the M standard (Some SA and K certified helmets may not be street legal).
The SA and K standards include a rollbar multi-impact test while the M standard does not.