I prefer the heavier bag. I find the lighter bags move way too much when punching them and you spend time waiting for it to come back into position or chasing it down to continue punching. I personally feel it is more realistic in effect since anyone you are facing in the ring is likely going to weigh well over 100 lbs. Plus, punching a heavier bag makes me use better punching technique with wrists locking, keeping a good line between fist and arm as to not twist things and injure a joint. I got sloppy one day and was working on the double left hook--one to the body then up to the head, and I got lax with technique and twisted my wrist on the head strike. It kept me from striking the heavy bag for a couple weeks before it healed.
But he's not going to stand right in front of you and let you bang away at him. When the lighter bag is moving around, don't just stand there and wait for it. Move in after it, hit it a few more times, then step to the side and let it swing past you and go after it again. Imagine it as an opponet that is moving towards and away from you. I like to do several rounds on three different bags. A heavier one (not sure exact weight) to practice power punching. A lighter one that swings around to practice speed as well as footwork moving around the bag. I'll also use a wrecking ball style uppercut bag to practice head movement, hitting it and then rolling underneath.
