A difference of perspective maybe: seeing your lower body as something that needs to be taken in hand and disciplined versus seeing your lower body as source of possible strength to power you though your cardio activities and any sports you play.
When to use each one? Can't think it makes much difference really. Whenever you feel like it. But typically, I don't like to involve cardio in strength building. I find that high intensity lower body training typically has its own cardio factor.
Clare