The mentally toughest team will win.
The Mavs could win if they attack relentlessly. The Spurs weakness, as it has been since David Robinson retired - excepting when the Spurs had either Kurt Thomas or Robert Horry in the game and paired with Tim Duncan - is in the dead center of their defense: exactly in the center of the lane and at the lip of the cup. The very heart of the Spurs defense is the weakness of their defense.
Do the Mavs have the heart to relentlessly, unceasingly attack the heart of the Spurs defense? Probably not. If the Mavs can find the heart: the Spurs are there to be had.
The Mavs weakness is their over-reliance on their 2-3 matchup zone. It's a nice change of pace defense, yet the Mavs have been relying on it for huuuuge chunks of the series. The Mavs can't win that way, imo.
Its a good series, and at this point is unpredictable. Some NBA playoff series are ruined by NBA referees refusing to allow an athletic shooting guard such as Kobe, LeBron, or Dwayne Wade to be defended. This series is not marred by that. Ginobli doesn't have the athleticism which convinces NBA refs that he simply cannot be defended w/o being fouled. Makes for a more interesting series.