The big differences that jump out:
KO2 can sequence external gear via MIDI, has both live and linear midi sequencing, onboard mic.
20 seconds max individual sample time, fun and fast.
SP404 has 16 mins max individual sample time and huge memory via SD. Both live and XOX style sequencing that might be more inscrutable vs the KO2’s workflow. USB audio and MIDI makes grabbing samples off devices and integrating with mobile music making way more convenient. Has loop, resampling, reverse, more FX types. No ability to midi sequence other gear. Overall probably much more versatile than the KO2, though maybe at the cost of an intuitive UX.
The two of them together could be pretty nice, e.g. using the SP as a multitrack to record stems jammed in from the KO2, resample on the SP with FX and layers, then maybe bounce back to the KO2…yikes.