Ginobili has been the Spurs ultimate X Factor over the past 3-4 years. 05, he had one of the best postseason runs I've seen. 06, even though he was technically the one that people will point to who cost us a repeat with that Dirk foul, it was also his 3 point shot that led the Spurs into a tie and back into the game after being 20 down, 07 though Parker outplayed him in all 4 games of the finals, Ginobili was the X Factor against the Suns and Denver. 08, a healthy Ginobili would have beaten that Lakers team.
I love Parker and he's obviously the more consistent player, but in terms of the player who's had the bigger impact on the Spurs success these past few seasons, it's gotta be Ginobili. People are going to remember him a lot more not just for his clutchness over the years, but for the style of basketball he's brought to the Spurs, his unselfishness switching to 6th man, and the fact that as I mentioned, he's the ultimate X Factor the Spurs have ever had in the history of the team, I think.