That's largely due to having out own integrated Air Wings, and how closely the ground guys train with the aviators. The army is somewhat handicapped by having to rely on a separate service, the Air Force, for much of its support.
That's not just an old jarhead saying that. A family member who is a major in the Air Force, and who has worked extensively with those issues in Iraq, is eligible for his Command level school. He's elected to go to the Marine Corps' Command and Staff College in Quantico precisely because he's been so impressed by how the effectively the Corps coordinates its fire support. btw, some Marine arty battalions are armed with the M109-series rather than the 198's.
I'll agree on the integration part, but mostly for fixed and rotary wing aviation. I still think the Army's direct and general artillery support is more advanced. A lot of people want the Army to own the Air Force's A-10s, but that would be a mistake. Having to rely on prepared airfields would be a step in the opposite direction. If the F-35 VSTOL doesn't make the cut, that will become awfully apparent.