Ever notice the heavy lifting in Iraq, the house-to-house, kick in door stuff is done by Marines? The Marines were the first large combat force into Afghanistan also. At the time, I seem to recall Rumsfeld saying the Army wasn't combat ready.
No I can't say that I have heard that the Marines were doing the heavy lifting in Iraq. Marine units in Iraq all have US Army soldiers working with the Marines in the field and the Army units do the heavy fighting.