For muscle gain, you are trying to work for more total weight lifted. You are actually doing more work when you bench press 200lbs. 10 times than if you did 300lbs. 3 times.
200lbs. 5 sets of 10 = 10,000lbs. Total Weight Lifted
300lbs. 5 sets of 3 = 4,500lbs. Total Weight Lifted.