For a purist and better player, the K90 by a long shot.
For a serve and volleyer or someone looking for something easier to use, the PS90 by far.
The ONLY advantage the PS90 has over the K90 is maneuverability (and less feel if you prefer that, but I consider that a disadvantage). It's actually significantly better at net.
The K90 is more stable, has more power potential, and has better feel. But the PS90 is literally just stable enough for the net.