Phalanx, Goalkeeper and Millenium use different approaches to combat their target.

Phalanx throws a stream of either 60 or 100 penetrator slugs (15mm DU AP, sabotted) at the target in about 1.2 seconds.

Goalkeeper fires about 15 penetrator slugs (the same 15mm long-rod penetrators as Phalanx, in DU or tungsten, but with more power and speed) at the target in only 0.2 second long bursts, unlike Phalanx with automatic kill assessment after each such burst. This is different from Phalanx in so far as it truely enables combatting multiple targets.

Millenium places typically 18 small penetrator clouds in its path (total ~2700 55-grain tungsten AP bullets per burst) within 1.0 seconds.

USN tested Goalkeeper in 1990 as a possible successor to Phalanx (CIWS-2000 tender). Goalkeeper did not show sufficiently higher performance than Phalanx in these tests. Hence the whole CIWS-2000 program was cancelled in favour of upgrading Phalanx. This coincided with Philips selling Signaal to Thomson-CSF, adding a political angle. The CIWS-2000 tests with Goalkeeper involved 12 different targets (Harpoons, Exocets, target drones), all shot down by GK.

We need also consider reliability (mtbf, stoppages + duration)