Both are "snell" rated. But SA is for Competitive
Automotive Sports
SA2010 - "The capacity for impact protection is determined by direct measurement of the shock delivered through the helmet to a head form when the helmeted head form is dropped in a specified manner onto any of three unyielding anvils. A fourth anvil is used to test impact protection for
repeated strikes against a roll cage assembly."
This is what wparsons is saying.
Edit - M does not seem to have
repeated strikes test.
M2010 - "The helmet must also resist penetration by sharp edged and pointed projections and projectiles. This capacity is tested by placing the helmet on a head form and dropping a metal cone of specified mass and geometry onto the shell. The tip of this cone must not penetrate to the head form."
http://www.smf.org/standards/sa/SA2010/sak2010_final
http://www.smf.org/standards/m/2010/m2010_final