The MPC has a better build quality and a better keybed, and is more for the people seeking some sort of modern ASR-10, and of course it’s more flexible regarding effects and routing.
The Fantom-06 in my opinion is a better workstation, and better stage keyboard despite having an inferior keybed because it has a ROM, thus changing presets doesn’t incur some sort of long blank unlike for the MPC and Fabric XL. The Fantom 06 also has a better sampling engine with more settings, doesn’t have the instrument plugin limitation the MPC has because subtractive synthesis is completely integrated with the regular patch engine, the Fantom 06 has more controls, a much more modern sequencer with an actual step-rec mode which the MPC lacks, and of course it has MIDI clips for jamming which the MPC key lacks.
Finally, the Fantom 06 has audio over USB, which means the musician can track out each track directly to a DAW, the MPC can’t do that. The MPC can’t even do USB MIDI without some network driver…
To me it’s not even up for debate, the Fantom 06 is a better workstation than the MPC key, because the MPC key wasn’t designed from the ground up as a modern workstation, it’s just a MPC Live with a keyboard attached to it and you can’t even have the pads control 1 instrument and the key control a separate instrument independently, they will play the same selected track (or performance but I shouldn’t have to set up a performance to do such basic thing). No problem via MIDI out with a remote keyboard though…