Another answer to the A-10 v. Apache question:
Commenter bigfoot got it right: one reason that the Army doesn't have A-10s is that it isn't allowed to. The Army tested a few jets in the late 1950s and early 1960s (including the Northrop N-156F and the Hawker XV-6A, precursor to the Harrier) before realizing that the USAF was not going to let it have them. After that, the Army worked on armed helicopters and the USAF - having adopted the Navy's A-1 in Vietnam - started down the road that led to the A-10.
(Photo: Northrop N-156F (it wasn't called the F-5A then) on an Army evaluation in 1961.)
Today, almost anything can do Close Air Support (CAS). The keys to modern CAS are sensors, communications and weapons rather than the aircraft. For instance, the US Navy is using the F/A-18F as a forward air control - airborne (FAC-A) platform, exploiting its Raytheon ATFLIR targeting pod, helmet-mounted display and datalinks. In that role, the F/A-18 pilot can look at a potential target, automatically slew the ATFLIR on to it, lase the target and thereby determine its GPS coordinates. The airplane's computers can grab a still image of the target, together with its location, and that picture can be relayed to an air controller on the ground. The controller can then clear the fighters to attack (or yell "THAT'S US, YOU MORON!"). The FAC-A can pass the coordinates directly to any other aircraft that carries GPS-guided weapons.
Where the aircraft characteristics make a difference is in responsiveness and survivability. The helicopters travel with the ground troops - but recent experience shows that they are vulnerable to a lot of threats that fast jets can ignore. Fast jets, though, run out of fuel quickly, and it takes a lot of airplanes to maintain cover over the battlefield. That's where the A-10 shines, because the original idea (in 1970, when the requirement was written) was that the A-10s would loiter on "cab-rank" patrols behind the front line, surging forward to engage targets as required.
The A-10 also packs a formidable gun. Guns are back in favour for CAS, because of their precision and because of their low risk of collateral damage.
Taking the endurance theme even further, the Army has tried to sneak one past the USAF with the Sky Warrior UAV, which is nominally an intelligence, surveillance and reconnaissance asset but will eventually carry up to eight Hellfire missiles. The USAF has detected this move and is trying to take the program over.