In Brazil, Mexico, Cuba and etc they normally use bag gloves. In USA normally is used boxing gloves for heavy bag.
Now, it does not have the best one for you, the best is that one that you fell better. But do not hit a heavy bag with your boxing gloves than go fight with it. Heavy bag is just for the bag! And the best choice would be a 12ox boxing gloves.
I personally use bag gloves for bag and bonxing for fighting.
And if you want to get in the MMA sport try to use the CENTURY LEATHER BAG GLOVES... it is awesome!!!! Anderson Silva uses one!
Try this shop! www.mmashop.com.br