Yeah, assuming it's actually drawing 300 watts then it will be heating 300 watts. But if something like the element is heating the body of the heater and making the thermostat read high it's possible it could switch off early until it cools down and reads accurately again.
Seems it wouldn't be too hard to test. Throw one of each heater in a cold tank for 2 hours, put a watt meter on them. Both should peak at 300 wattsish and use .6 kw/hours.