Perception versus reality. An interesting study published in the Journal of Strength and Conditioning Research compares how subjects rated the difficult of their training with the number of calories the effort actually burned.
Nine active college aged men performed a circuit of 15 reps at 10 different weight training stations using 40% of their one rep max. Then they ran on a treadmill at a speed that would equal the heart rate they reached while circuit training.
Although they burned an average of 168 calories during the circuit training workout compared to 244 calories while treadmill running, subjects rated the circuit training effort a more difficult 6 on a scale of 1 to 10 compared to the much lower 4 rating they gave treadmill running.