Your premise is skewed in my opinion. I don't know why, other than custom and the anecdotal information from a lot of bad shots, why you'd say a 20 gauge slug is "marginal". A 20 gauge round ball (325 grains of lead) launched by 80 grains of black powder (three drams) was the preferred deer and moose gun of Canada until the cartridge guns became readily available. A modern 20 gauge, even with a smooth bore, cylinder barrel, plus the rifle sights, can be very accurate to 50 yardss, with the right slug, and if you're offering a scenario with a rifled slug barrel, the 20 gauge will do just fine. Nothing marginal about it at all.
Where I live the answer is simple, the 20 gauge, because where I live
in the People's Republic of Maryland, using the .357 lever gun and the .223 bolt action rifle, is prohibited for deer, period.
So I'd test fire different brands of sabot slugs, but I'd bet 15/16/ oz. Rottweill slugs
https://www.sportsmansguide.com/productlist?k=20+gauge+sabot+slug or 7/8 ounce Lightfields
https://www.sportsmansguide.com/pro...xp-20-gauge-2-3-4-7-8-oz-sabot-slugs?a=709632 would be accurate, or if I was using a smooth bore "slug barrel" with rifle sights I'd opt for original style slugs from Rottweill
https://www.sportsmansguide.com/pro...e-275-shell-15-16-oz-slugs-5-rounds?a=2108589. In fact I use the last version of the above mentioned slugs in my 12 gauge 870 with a smooth bore, cylinder choke barrel with rifle sights, and out to 50 yards (since that's the farthest shot I'm going to get) and it's very accurate. I got the "slug barrel" cheap for my 870 goose gun, because I had a 12 gauge 870, but I'd still hunt with it and that barrel and slug if it was a 20 gauge.
One of the most accomplished hunters I've every known in person, used a rifled, 20 gauge shotgun as his "meat gun" for shooting does when he wanted venison..., now Bill put a 4x scope on his to go to 100 yards, but my eyesight was way better than his when I knew him.
LD