1) For training bonuses up
Light training is better. More bang for your buck.
2) For skill improvement/SA gaining vs condition loss
No difference between hard and light training. 2 factors are key. Firstly the current rating of the attributes in the drill. The lower the attributes, faster improvement and gaining. Secondly, the training level of your drill. World class vs 1 star makes a big difference.
3) To save time
Hard training is only better for speed. It will train your players much faster than usual because of the intensity but it might not be as cost efficient as other drills that might have a higher training level or lower attributes.