Both connector families carry PCIe signals over a cable instead of across a backplane. They differ mainly in density and in the generations they are intended for.
OCuLink has been widely used for x4 links to drives and external devices in earlier PCIe generations. MCIO is a newer, higher-density family that is designed to support higher data rates and wider links, which is why it appears in current server platforms for cabled PCIe to drives, accelerators and expansion boxes.
If you are designing for a recent platform or a higher PCIe generation, check which connector the platform specification calls for and confirm that the connector and the cable are rated for your target data rate. Cable length and the loss budget matter more as the data rate goes up, so verify the full channel and not only the connector.
