Verdict
K1 Max gives you a 300×300×300mm cube and Klipper firmware at $749. P1S is 256mm cube but enclosed with AMS support at $699.
Buy the K1 Max if: you'll print parts over 256mm regularly, value Klipper customization, accept higher tuning effort.
Buy the P1S if: mostly mid-size parts, multi-color via AMS matters, you want Bambu Cloud polish over Klipper flexibility.