Hmmmm. Too easy. As a couple of other folks have said, depends on the mission or overall objective. Take Desert Storm for example. 5th and 3rd Group Special forces, a Patriot battery, 82nd Airborne ready company, IRC, DRF and Flyaway from 24th Infantry Division (Mech) Ft Stewart were the first to deploy. This is the XVIII Airborne Corps DRF component. The mission is to arrive anywhere in the world in "LESS" than 18 hours. When you are on hot status, your equipment and personnel remain ready to deploy. Aircraft are also on hot status to deploy troops and air transportable equipment.
Marines however, are usually "easier" to deploy to coastal regions due to already being at sea. Again, depends on the mission and enemy target expectation. Iraq had the 4th largest army in world with large amounts of tanks. Marines at the time only had one or two companies of M1 Abrams and the rest were M60s. So XVIII Airborne Corps and Special Forces groups got the nod to be speed bumps until the rest of 24th Infantry Division could arrive days later. The entire division was in country and set to fight before any marine touched sand.
Now let's reverse that. When the uprising in Beruit began, Marines go the nod due to the limited scale mission. Same thing with the initial deployments to Somalia and Grenada. Panama was a dual deployment but I believe Rangers were first on the ground along with 3rd group. Then everyone else.
As a general practice, and I could be wrong, even after 22 years of service, but the XVIII Airborne Corps is still the nations response team "WHEN" heavy resistance or mechanized forces are expected. If the expectation is your common infantry and maybe truck born weapon systems, obviously marines already afloat are more suited for the task. But the modern battlefield is dynamic. Rest assure that no matter what th enemy threat is, the proper unit is no more than 12-18 hours away from putting rounds on target. The March on Baghdad during OIF I saw some units from the 3rd Infantry Division (Formerly 24th infantry division) deploy from nearby Kuwait in which they were already there doing training. The middle east belongs to Ft Stewart and Ft Benning, Georgia. Been that way for as long as I can remember. This is why our vehicles were painted sand color for ages even before Desert Storm. And the reason coalition forces painted V's on their vehicle because Ft Stewart again, has always had V's to denote not only friendly forces, but which unit you are looking at. ^V<> all those V's mean something and commonly it means Alpha, Bravo, Charlie, Delta........ And when you put DOTS underneath the V, it tells you which platoon in which company the vehicle belongs to.
Army Rapid Deployment forces that include DRF (Rapid Deployment Force), IRC (Individual Ready Company), IRB (Individual Ready Brigade), Fly-Away (C-130s, C-141, C-5, C-17) and RC (Ready Company from 82nd Airborne) are always on standby 24/7 to deploy and defeat any ground threat known to man be it tanks, infantry, artillery. The DRF component also deploys it's organic Air Defenses which currently is Patriot Battery and Stinger/Avenger teams. Marines are no longer the Marines of old. They have become a more stategic and technical option to apply a surgical force where needed. But the Heavy hammer that is needed to deter "Further" aggression will always be the mission of Army DRF units. Once they have established a defensive phase line, then your Marine forces are arrayed to either compliment that array of forces, or maneuver to posistions to either exploit whatever gains the Army has made or establish a secondary front so that both forces can screen each other's flank. Lastly, it's not a dig against my marine brothers, but I have never deployed to a region where marines were already there. And I have 7 tours in combat in various hotspots. But then again, I know for a fact marines have deployed to some spots that the Army was not there, nor did they even deploy. The exception being 5th and 3rd group. Take Lebanon for example. Primarily a Marine and Air Force mission.