The 2kw is the maximum output, so in theory they will both be using 2units per hour if they are running at full pelt (because a unit is 1kw for 1hr). What they actually cost depends on how much time they are running, and that depends on your thermostat, and where you put it. And how effectively your greenhouse is heated will depend on how effective the heater is at spreading the heat around the greenhouse, and where your thermostat is. For example, if your thermostat is in a draught from the door, it'll be keeping the heater on a lot longer than needed, whereas if it is just above the heater, it'll be thinking the greenhouse is warm enough when areas further from the heater are still chilly.