So what load were you using in both rounds?
In general a 12 will pattern a little better than a 20 with the same loads, which is a general rule with ANY kind of shot--though the difference is less with harder shot.
My wife and I have killed a pile of turkeys with both gauges using 1-1/4 ounces of #6 and #5 shot (both lead and Bismuth) out to 40+ yards with no problems.
