I was reading an article on AZCentral and I think they gave the stat that will decide the series:
"The Spurs went 2-12 when allowing 100 or more points during the regular season."
So that's the key. The Suns score over 100 points in the game they have a good chance of winning. They don't? The likelihood of the Spurs winning the series goes up.
I guess we'll see how the first game goes to see if the stat holds true. During the regular season, the Suns scored 100 points against the spurs twice and lost once in overtime. The scored less than 100 against the Spurs once and they lost that game.