I am no personal trainer but I've spoke to quiet a few. Low reps, heavy weight is good for gaining mass or strength. High reps, medium weight are good for cutting and endurance.
Honestly from my own experience you should always be mixing it up. Go a few weeks with low reps, go a few weeks with high rep. Even better if you try and finish your sets with a drop set (on your last set do as many as you can, then half the weight, and then repeat).