The 350789-3: The mating contact finish is 30.0 µinch (0.76 µm) gold over phosphor bronze — this is a robust plating grade for repeated mating cycles.
The 350789-3 is RoHS compliant, so it fits into lead-free assembly lines without exemption paperwork.
The 1318699-3 is a 12-position, 4-row header at 4.20 mm pitch — it carries more circuits in a denser footprint but uses only 3 µinch gold on the mating face, which limits reconnect duty compared to the 30 µinch gold on the 350789-3. The 350789-3 is the better choice for a low-circuit-count, high-cycle-life application; the 1318699-3 suits higher-density boards where the connector is mated less frequently.