from what I have read and watched on tv on the history channel and other channels that talk military stuff.
USMC is the quick rapid reaction force, can send them any where and set up off shore and wait for the order to go.
Fast operations, hold till reinforced or tasking is over.
Have enough force projection to take, seize and hold ground by force,have their own small airforce, heavy lift , troop lift and gunship helicopters, good size tank force, arty , everything they need to do the job. Large support ships run by the navy for on shore and off shore support, guns, air support and medical support. Can move all of their own equipment by navy ships and navy aircraft.
they call themselves Uncle Sams 9 1 1 force.
have the nice dress blue uniforms. no uniform in the US forces beats the Dress Blues, international symbol of Us power.
Army.
larger force, needs a land base to jump off from, cannot sit off shore and wait for the word to go
has larger troop levels, larger tank force, arty, no armed fixed wing airsupport, needs airforce for anything other tac hel, has own medical support, needs the support of the other 3 forces to move equipment.
those are just my thoughts of the topic.