What are you using it for? If you're just using it to exercise, the 100 lb. one might better because its going to be more stationary since it requires more force to move it, and if you're just exercising you are going to focus more on moving yourself than hitting a moving target...but 20 lbs. isn't going to make that much of a difference really.
For actual training, I guess the first one but since you are going to need a training partner I would suggest maybe just joining a fighting gym
If they are the same price (I didn't look), I guess I would go with the 80 lb one. If you decide to attach a weight, I wouldn't do it with a chain. Secure it firmly. Also make sure you don't knee it, or if you do make sure you have video so I can laugh at you later.
Otherwise I'd go cheaper cause I'm cheap.