Why do you ask? What do you intend to do with the metals? Weld? Bend? Form? Machine?
And where do you intend to use them? Roll Cage? Frame rails? Floor pan? Sheet metal patches? Formed sheet metal parts?
http://www.suppliersonline.com/propertypages/300M.asp
Apparently they are about the same material, from the link above:
"Principal Design Features
300M is a low alloy, vacuum melted, steel of very high strength. Essentially it is a modified AISI 4340 steel with silicon, vanadium and slightly greater carbon and molybdenum content than 4340. 300M has a very good combination of strength (280 to 305 ksi), toughness, fatigue strength and good ductility. It is a through hardening alloy."
http://www.suppliersonline.com/propertypages/4340.asp