A Japaneese research group did a study that concluded the 10-12 rep range was optimal for gaining mass. This is because the extra reps help raise the bodies natural GH levels by as much as 30%. The 6 reps range has been proven more effective by scientific study to improve strenth. I have noticed it is also hard to get a good pump by going low rep. To keep it equal I do a heavy weight low rep week and then a high rep 10-12 range pump week. Then repeat. How could you go wrong bro?