You need a combination of everything mate. High reps, moderate reps, low reps. Mix it up. If you have the strength go heavy, if you're sore from your last workout go moderate or high. Listen to your body, only your body can tell you what it prefers. Experiment and see what works best for you. If you get results from high reps, do high reps. If you get results from low reps heavy weight, do low reps heavy weight.