It is hard to determine anything based on regular season.
IMHO, the key to the series will be in the Cavs can get Duncan into foul trouble. This is not out of the question, since LeBron can get through any defense to get to the basket or force virtual triple teaming. But making Duncan stop LeBron is very dangerous for the Spurs.
So far, no one has really tried to create an isolation to force Duncan to defend a really athletic big man. Most players make the mistake of trying avoid contact, which all but invites Duncan to block shots. LeBron has shown an ability to get to the defender's body at an angle (avoiding a charging call) which prevents the defender from jumping and leads to fouls being called in the "and one" situations.
At times it seems like Duncan is immune from foul calls, but at the same time, teams don't really attack him like they do Amare. But when Duncan gets into foul trouble, the Spurs collapse like a house of cards with limited offense and vastly weaker defense.
IMHO, the Cavs only chance is if they can get Duncan into foul trouble. Otherwise, they don't match up well.