Huge differences between the two rackets. I played with the KPS88 in 5.5 level play since it's sale date until a few months ago where I decided to sell the whole kit. Prior to the KPS88 I used the K90.
IMO the KPS88 does not have the control that the K90 or the PS85 have. In lower level play, control was not an issue. But the heavier the ball became from the opponent the more a polarized setup was beneficial to point construction. Although the PS85 does not share a polarized setup, it however is extremely quick in getting the racket in position.
Although I could rely on very deep placement and good court penetration with the KPS88, I could not trust the directional targeting if I was slightly out of position, especially in higher level matches. The KPS88 straight away is a ball basher. It is a true muscle car. Very fast in one direction.
The PS85 and K90 are more elegant rackets. Think of a performance sports car. I found myself to be significantly more successful with serving from these two rackets than the modern pro staff. In addition, I was able to create a greater variety of game.
The KPS88 when hit squarely was an incredible feeling. You felt the ball come off your racket like any Apollo mission. I think that's why I stayed with it for so long. Alas, a heavy kick serve that came bucking like a mule was not easily manageable with the non-polarized KPS88. The K90 was more adept at handling restless beasts. And I feel the same for the PS85 because of its quickness.
The power of the KPS88 is addicting. But after rehab, The subtleties of point construction became a familiar sponsor, hence the return to the K90.
I have also tried the BLX90. I prefer the K90.