Based on what you said, and if you are shooting 9TSS, start with either a .570 IC or a .585 Trulock Precision Hunter. Both have shot lights out over and over and are slightly more forgiving as you said you are seeking.
If going lead, I would go with a Carlson .568 or maybe an IC .562. Lead is just a bit harder to find what you like in a 20 gauge. The smaller gauges often begin to tell on itself with lead shot.