The Heat are a good team with some great players. I don't know if they are a better team but won't argue that they are playing better in the Finals.
The Thunder destroyed my Spurs by making the extra pass, playing team ball, and getting big (and uncharacteristic) contributions from Sefolosha, Ibaka, Fisher, and the rest of "the others". In the Finals, it has been only Durant and Westbrook with occasional sightings of Harden. On the other hand, "the others" on the Heat (Chalmers, Battier, etc.) have really stepped up to help out a very determined James and a struggling Wade. It also seems like Bosh is back into the flow of the game.