The difference between them is similar to the difference between these: 1. I am listening to music (present continuous) => It indicates that the action is happening right now. 2. I listen to music (present simple) => It states I sometimes listen to music, i.e. my general habit. Similarly (except that they are now the future perfect continuous and future perfect simple tenses), 1. he will have been training the horses for 5 hours => emphasizes the continuous activities that will have gone on by that time. 2. he will have trained the horses for 5 hours => doesn't have the "ongoing" sense, so it sounds like it's stating the fact of training and the duration of the activities which is 5 hours. So I would say #1 makes the sentence more effective description of Hank's work.