Recommendations depend on what shell you plan to shoot and whether you intend to use an optic. I shoot a 686 12 gauge ultralight and handload TSS 9s (1&5/8oz) for the long barrel and 1&1/4 oz TSS 9s for the short barrel. I also use a B-Square cantilever mount for a Burris FF III red dot.
For a single barrel gun, I want a 50/50 pattern, e.g. 250/10" and 250 in the 10-20" circles to provide some wiggle room. For an O/U, I want the "long" barrel tighter than that. Mine with a Sumtoy .675 does 350/10 and about 180 in the 10-20" circle. I am currently using a flush factory full in the "short" barrel. If you want more wiggle room with the "short" barrel you can go to a modified or even more open or shoot lead in that barrel. Your gun will likely shoot closer POIs between the two barrels if you shoot the heavier load in the bottom barrel. In most cases, the lower barrel will shoot to a lower POI than the top barrel if the loads are the same just due to the geometry of O/U guns.
Indian Creek is a solid choice as is Sumtoy. Most guys are shooting .555, .562, or .570 in Indian Creek or .562 or .570 in a Sumtoy in 20 gauge. All of those will shoot killer patterns with TSS.
Good luck!
