Open collector is an implementation wherein the IC will not provide the supply by itself the external circuit by floating its collector and let the external pull up to provide bias on the external circuit. Push-pull on the other hand is an implementation wherein the IC is set to supply the external circuitry.

When the transistor is off, the output is internally disconnected from any internal power rail, a state called "high-impedance" (Hi-Z). Open outputs configurations thus differ from push–pull outputs, which use a pair of transistors to output a specific voltage or current.

In short, good advice would be that sensors that provide open-collector output configurations should be used with PLC’s or machines that are designed to accept open-collector input configurations, and sensors with push-pull output configurations should be used with machines that are designed to accept push-pull input configurations.

By eliminating the pull-up resistor used in open collector outputs, push-pull outputs use less power and are therefore good options for battery-operated designs. For only a modest price premium, push-pull output drivers provide an extremely flexible solution that is compatible with most systems.