I have a CP73. The keyboard is the BHS action - balanced hammer standard. To me, it sort of feels like a Montage 8 or XS90 or Motif 88 without the aftertouch (I also have an XS90). I don't own a CK88 but it has a GHS action - graded hammer standard, which is used in the MOX8, MOXF8, and MODX8 keyboards. I tried a CK88 out at the store and it felt ok but a few years ago I had a MOX8 backlined to me on a tour for a month. I got used to it but I didn't like it - it felt a little mushy. Maybe they've improved the GHS action over the years but I prefer the action of the CP73.