A recent probem in so many FPSs is that developers are obssessed with, "unscriped events" events that are brought about by the A.I., the players actions and in some caes, the game's physic's engine. When unscripted works, it works brilliantly, Halo 1 showed how good facing a slightly new challenge each time you played through a level. However one area of Halo 1 was that it seemed Bungie was incapable of a good set peice, then we all saw the last level and one of the biggest, most explosieve and action packed scenes in gaming ever. The warthog run in The Maw was astounding and gamers everywhere wondered, how will Bungie top it for Halo 2? Having seen the E3 2003 preview of Halo 2 with a massive battle on earth, it seemed that Bungie had learned not to rely on the A.I. but also plan out large battles. Halo 2 looked like it would be the best console action game ever, and there was talk it might even move in on Half Life 2's turf with best overall FPS. Weren't we all in for a shock.
November 9 2004 came and not only was the New Mombassa level from E3 removed, it seemed that Bungie had ditched plan action scenes for their A.I., player interaction. Admittidly there were huge battles but they felt to chaotic and lacked the planning of what many hoped for, it also removed any sense of speed from them. Meanwhile GTA: SA apperead with numerous missions including large scale set peices and GTA went on to claim the console crown, with Halo 2 taking 2nd place, just though.
Don't let this fool you, Halo 2 is indeed a close 2nd, in many areas it surpasses GTA, graphics, A.I, and multiplayer, the reason that SA is a better game is that the gameplay and sheer size put it above Halo 2. That said I'd still buy Halo 2 it if goes to PC. Its that good.