Verdict
Both are flagship CoreXY machines for production work. X1C is the more polished out-of-box experience with AI failure detection and AMS. Core One is Prusa's first CoreXY — open-source firmware, modular design, slightly higher build quality per unit.
Buy the X1C if: you want fewest setup decisions, multi-color via AMS is non-negotiable, plan to scale to a Bambu farm.
Buy the Core One if: you'll keep this machine 5+ years, value Prusa's parts/serviceability ecosystem, prefer open firmware over closed.