Materials of construction. The housing and impeller can be either cast iron, bronze, plastic, or stainless steel. There is little advantage between cast iron, bronze and stainless steel for these applications. Cast iron is fine unless there are unusual corrosion factors. Plastic pumps are perfectly acceptable but are less tolerant of sand wear than cast iron and plastic impellers are easier to damage or strip off the motor shaft.
Plastic pumps are light and corrosion resistant.
They tend to be lower end pumps but can be very cost effective. This is not something to worry about in this class of pump. There are other differences but these are the ones to be concerned about. The basic function is lifting human, or animal, waste up into a disposal system. Human waste volume ranges from 50 to 100 gallons per person per day.