I myself would go with the 12 gauge unless like the earlier poster stated your concerned about recoil because yourelderly or have intentions of your wife or one of the kids to use it.
My son and I hunt together, we have 12 gauges and it makes it handy that we have the samesizein case somebody forgottheir shells at home or whatever. So that might be a determining factor.
The 20 guage will more than do the job, but like I told my son as wereload our revolvers with heavy bullets for the upcoming season, why throw a tennis ball if you can hit the same spot with a bowling ball?

Dan