Having served in both, there is very little actual overlap. The overall missions are the same, but unlike what what other users are saying, the Pentagon most certainly choose units based on Army or Marine orientation.

Why? The Army has failed, as a whole and in parts to be considered combat ready, because even most infantry units were unable to meet qualifications. The Marines meet every benchmark, in whole, and in most parts.

Why is a difficult beast, though the Pentagon came to an interesting conclusion, a part of which was training. By streamlining training, even support elements for Marines are combat ready, whereas Army support units are not qualified to operate independently.

The effect of this is, areas of higher combat get either specialty Army units (10th Mountain, etc) or Marines. Additionally, when Marines are selected, the support elements needed in that theatre are brought in and made organic to that element, because support elements in the Marines are interchangeable, whereas Army units come with already attached organic elements that are NOT interchangeable.

This then leads to Marines being chosen almost exclusively for heavy combat roles, or rapidly evolving situations. After all, this is the role they have been designed for, and is the reason their Org charts are the way they are.

This doesn't make them better (though my experience is Army infantry in non specialized roles is poorly trained), it just makes them more capable to handle a lot of situations.

Having gone from front lines combat with the Marines to Bn HQ with the Army, it was interesting to learn WHY we got the AOs and missions we did.

TL/DR, Army primarily was used in daily routine missions and reconstruction, Marines were used in areas that were rapidly evolving and heavy combat due to their modular organization and rapid response roles.