I'm leaning heat on this...
Maybe not. In their series against the Pacers, they barely won Game 1, and the Pacers took them to 7 games. So, as a team the Heat isn't invincible.
Now, the Spurs have won Game 1 in Miami, a good first step. In fact, I think it's a necessary first step. You don't want to spot the Heat the first game.
The problem the Spurs have is that 2-3-2 format -- especially that 3-game series in the middle. It's almost impossible to win 3 in a row against a good team, and Miami is a very good team. So, the Spurs winning Game 1 doesn't really give them home court advantage if you assume they won't win 3 straight in SA. Assuming that, the Spurs will need to win another game in Miami.
The best path to a San Antonio title, assuming the Heat wins Game 2, is to let that game give the Heat a little more confidence than they should have and ease off a little, and then the Spurs do take 3 in a row in San Antonio, or go 2-1 and win game 6 in Miami. A game 7 is likely to go to Miami at home. (A Game 7 loss for Miami at home would be considered a stunning defeat.)
No matter what, for the Spurs to win this series, they'll have to win more than one game in Miami.
Steve.