Excluding all the technical terrain arguments; standing on smooth surface climbs will use more energy. What I found however, is that my heart rate goes up (more energy consumption), but some muscles gets a rest. What I do sometimes, lean more on your arms/shoulder and use your weight to pedal down by bending your legs as little as possible. More of the upper body muscles gets used, which is propably the reason for the increase in heart rate. This allows you to have a upper body workout and resting the legs by using rested muscles and gravity in the pedal action.