Softer bags are better for your joints. I remember reading that Larry Holmes got wrist problems out of hitting a harder bag. Soon thereafter he switched to a soft bag.

If your bag doesn't swing much no matter how you hit it, then don't hit so hard, it'd be bad for your joints. If all possibal use a heavybag that hangs to where the bottom of the bag is on level with your waste. Just like Grey said don't hit on top the heavybag, and avoid heavier bags.