I feel you should incorporate both methods to hit the fast and slow twitch muscle fibers. It's good to shock the fibers by changing the routine every so often. I like to incorporate a shock day and do 100 reps of a given excercise as fast as I can under good form. Prepare to puke or pass out if done right! LOL
As for your question, you will not gain mass by only doing low weight high reps. You need enough weight to stress the fiber and not just pump them as to activate repair.