Had the chance a few days ago to try some S&B and some Sig .300 Blackout subsonic ammo. Both fired out of a AR the owner had built, all at a 30-yard indoor range.
four out of six almost one big hole. The problem I ran into was that when fired without a suppressor, it would not cycle the bolt far enough to pick up the next round. With the can it did, but not far enough back to lock the bolt when the last round was fired. This rifle had a adjustable gas block, I'd imagine that opening it up a click or two would take care of that.
Four of the five in one big hole, not bad, and a little higher point of impact than the S&B. To me it sounded a touch sharper/louder, and it did cycle the action with no problem.
By the way, 'sharper/louder' doesn't mean much, according to a guy outside both were quite quiet, I'd say outdoors you could've fired both with no ear protection.
Did one more test, this on the Strike Eagle scope on the rifle. It has a dot as the main aiming point with several stadia lines below. The S&B, using the first line below the dot, was on a 30 yards.
So: both were quiet through a suppressor, the powder seems to have different burning characteristics, both capable of good accuracy at this range. The S&B might require an adjustable gas block, depending on your firearm. I'd like to have more of the Sig to test, and I'd like to try both at longer distance.