Verdict
Same Nextruder, same Prusa firmware/ecosystem, same load-cell first layer. MK4S is the proven bedslinger; Core One is Prusa's first CoreXY (faster, enclosed, better with engineering materials). Both have ~2.1-2.6% failure rate measured.
Buy the MK4S if: mostly PLA/PETG, budget-conscious, prefer the proven design.
Buy the Core One if: you'll print ABS/ASA/PC regularly (the enclosure earns its keep), want CoreXY speed, plan to keep it 5+ years.