Since you are not used to the .45 caliber muzzleloader a shotgun might be easier to learn quickly. While a .45 caliber roundball will more then do the job, a shotgun slug is also very deadly. Both have about the same range, and if all you are doing is trying to fill a freezer for the time being, until you can get a rifle of your own, then the shotgun with fast follow up would be the way to go.

You might still want to get to a range and shoot both of them. I know a couple shotguns that while with shot there were real good, with a slug they were terrible...

Good luck.