Well, I'll follow up to my original post. I was in a similar boat to you up until a few days ago. I was looking for a nice utility shotgun, and I was impartial to the 20ga, but after looking at price, selection, and availability of the 12 and 20 ga ammunition I chose to go with the 12. What it came down to was, I had a better selection of ammo (from BB to 000-buck, to 1&3/8th oz cannon rou...errr slugs) in the 12ga cambering then in with the 20 ga. It also helped that the 12-ga stuff tended to be cheaper then the 20ga. (100 rounds of 12ga target loads: $20, 100 rounds of 20ga target loads: $23)
From shooting the 12ga with Federal bulk 100 round 1&1/8th oz #8 hunting/plinking load and Super-X 9-pellet 00-buck, I will say that the Federal stuff does seem to kick a little more then the buck, which is great, since that makes it very economical to practice with, at $.20 a pop versus $1 a pop for the 00.
So, that's my take. Now for HD, 20ga and 12ga are both excellent choices, but from an accessory and ammunition price standpoint, I do think a 12ga has a slight edge. Both are Swiss-army knives of guns, just it seems to me that the 12ga has a wider selection of tools then the 20ga.
As for my Mossy, the only aftermarket thing I'm doing to it is putting a 6-position stock on it so its more compact for storage (its my truck-gun) and because the LOP on the OEM stock is a tad too long for my arms. I am NOT advocating that you need a laser, high-intensity flashlight, Knoxx Spec-Ops stock, side saddle shell holder, verti-grip slide, an EOTech, and have the whole thing rail encrusted for it to be effective at HD, or that you ought to get a Saiga-12 with all the above with a 10" barrel and an NFA stamp. I personally find the simplicity and the implicit reliability of a pump much preferable to an automatic. Heck, the Saiga is about the only automatic I would trust, and even then not completely.
Happy hunting!