Giz has it right about slugs. I see very little use for slugs in HD scenarios. Go with some "Lossleader" ammo for basic breaking in and fun, and pick up some of those 5 packs of various buckshot. Now the work starts....
Hie thyself to a range or place where you can pattern that shotgun/load. Take a magic marker along to ID which load is which.Of course, by this time you've measured the longest shot possible in your house and added a yard(5 feet will do to, Giz). Pattern at that distance and fire at least three shots of each brand,one at a time and mark as you go. Then, pick the one that patterns tightest. For sizes, I'd check out 00 and 000, and maybe #1 buck also. While you're burning up all that ammo, try patterning some of that loss leader bird shot at that same distance. You may find that pattern interesting....
And, you may find that one or more brands do not hit as they should. Adjust sights, or if just a bead sight, adjust the stock to bring most of the pattern over the bead, IOW hitting a hair high.
Once you've determined which brand and size seems best, go buy enough to quell a small riot. Shelf life on ammo kept cool and dry is measured in decades, and chances are it's not going to get any cheaper.
Now, once you've got this part covered, and just HAVE to have slug capability, bench and zero with divers brands. As a loose rule, the Forsters and Brennekes do best in smoothbores, so check them out first...
Hope this helps...