Spurs in 5. They just have too much speed and defense.
Their defensive scheme (and the giant they have inside) has given problems to the big men for the Blazers (Clingan and Williams), Wolves (Gobert and Randle), and Thunder (Holgrem and Hartenstein) in consecutive series. I expect that to continue to with KAT and Robinson.
The Knicks offense was hot, but I think a good portion of that is good shooting luck, which should regress to the mean, and may regress lower against better defense.
The Spurs will rotate Castle, Vassell, Bryant, Fox, Harper, and Champagnie on Brunson. Brunson is a special player, but his game is not as good as SGA's. He may take/make a few more threes, but he won't finish at the rim as well and the Spurs should be well-trained against his foul-baiting.
I expect Fox and Harper to have big series. I also expect Wemby to be more successful inside against KAT, who has pretty bad habits against the pick-and-roll (I'm surprised the Cavs couldn't take more advantage of that last series).