Freezer,
I agree with the rest regarding the two gauges being equal overall.
However, I now regret that I bought a 20 guage barrel for my Topper instead of a 12, due to availability of ammo.
Where I live, I have been unable to find the much-cheaper remington sluggers, or brenneke's, which can be used in rifled barrels.
So I have a 20 gauge slug gun that I can't afford to shoot.
Yes, the 12 kicks harder, but bench shooting you can fix that with shoulder pads, and the like.
Maybe you already made your decision, its been awhile since your post, but if I had it to do over again I would go with the 12.