You don't list where you live so, I'll make a general statement considering what season of the year it is and what part of the country you live should be taken into consideration. In summertime, a bad guy wearning a T-shirt versus a biker in heavy leathers or a snow bunny bundled up will dictate different penetration needs.
Next, why in the world would anyone consider birdshot? I just don't understand that one. I've listened to a lot of totally uninformed people argue vehemently that birdshot would take take down a drug crazed Godzilla beast if shot at room distances.
If you can live with the penetration of a 12 gauge slug, you should consider using a rifle. A soft lead projectile isn't what I would want in a life threatening situation at room distance. Maybe a 45-70 or 44Mag lever action Marlin or, a nice Belgian FAL, even considered a 50AE conversion for my AR-15.
That pretty much leaves Buckshot. I want really hard shot so, lead is out. There are several different premium loads to choose from and, I have had good results with Copper-plated Buckshot. There are other hard and heavy materials to consider but, I haven't had enough time to work with them myself so, Copper plated it is.
Now, Buckshot size? I consider individual pellet weight, diameter and, penetration ability. Nothing smaller then #4 or larger then 000. In areas where I am concerned with reducing over penetration risk, #4's which will still be a threat on the other side of a sheetrock wall but, less so then large Buckshot. With 41 Pellets, you also get a lot of coverage and penetration is adequate for bad guys not wearing heavy clothing. I am now willing to give up some of the pellet coverage (count) to get a larger heavier deeper penetration pellet and have settled on 00 after trying out #1's.
Now, power behind the pellets. Magnum? Non-Magnum? Reduced recoil? 3 inch or 2 3/4 ( maybe 3 1/2's

)? You have to pick one you can really shoot well. Not flinch with, fighting recoil, etc. Can you shoot it, keep your eyes open and on the target?
Finally, how many loads have you patterned at various distances with this life saving tool?