Since I 'm feeling ornery, I 'm glad to see several "none of the above" choices...it will be MUCH easier to outshoot one of you guys with no firing pin!...I suppose in all fairness I should have included "It doesn't matter...let the action maker choose for me." as a choice.
I have been a party to testing this with a single barrel and action. We removed the stock and scope and set the rifle in a wood yoke to hold it and fired 100 rounds in the 12 position and 100 rounds from the 6 position across a chronograph. We did this twice and the velocities were just nearly the same. On one test the 12 barely won and then on the other the 6 barely won. There is just no real difference.
I would not turn around for the difference. If your favorite gunsmith insists on one or the other and the cost is close, just humor your gunsmith and let him do what he wants because it doesn't matter.