I assume that your Mossberg is the rifled barrel version. There are a number of fine performing sabot slugs that are deadly including the two already mentioned. Try as many different ones as you can afford. The one that groups the best out of your set up is what you want to be using. Any other good grouper will do the trick as a so called "back up".
Don't go to a "rifled slug" as a back up. These are made specifically for a smooth bore. These can perform poorly from and heavily foul a rifled barrel.