Verdict
QIDI Plus 4 is the value pick for engineering materials — actively heated chamber (60°C), 305mm cube build volume, 350°C nozzle. X1C wins on ecosystem (AMS, Bambu Cloud, polish), QIDI wins on raw thermal capability for PA-CF, PC, ULTEM-ish materials.
Buy the X1C if: mostly PLA/PETG/ABS production, multi-color matters, want polished software.
Buy the QIDI Plus 4 if: printing polycarbonate, PA-CF or ULTEM-ish high-temp materials regularly, need 305mm cube volume, on a tighter budget.