titanium it depends what you are training for, if you are in the marines and training for muscular endurance then higher reps will force your body to adapt to those demands
but the title of this thread was advice needed on "building muscle mass" which is what most people you will see at the gym are looking for, and to build mass you need to stimulate your body with as high a weight as possible, then eat and sleep enough to fuel the muscle growth. If you're doing 20 reps with something, it's not the highest weight you can use...probably 6-8 is about right.