Which costs less, 12 GA shells or 20 GA shells?
Is 20 gauge harder to find?
Is 12 gauge overkill for small game and upland birds ? Or does the type/size of shot in a shell decide that?
If you are purchasing your first shotgun and the intent is to use that one shotgun for everything i.e. deer, small game/upland and waterfowl including perhaps clay shoots then a 12ga would be the most versatile firearm. A wider selection of steel ammo will also be more easily available in most stores compared to the 20ga. Likewise, a greater selection of 12ga turkey loads are also more commonly available on store shelves.
FYI, felt recoil (dependent on the individual) will generally be substantial in the case of a 12ga slug. However, that is something experienced more during range sessions than on the field. For small game/upland, use of 12ga 1oz loads will serve fine and won't be overkill provided how that may be used in terms of shooting whether at a moving target vs. a stationary one.
Having said all that, a 20ga may also serve almost equally well if the shotgun is put to use bearing in mind it's payload limitations compared to its elder sibling, the 12ga. One could just as easily hunt deer, small game/upland (including Turkey) and waterfowl (effectively over decoys).
Generally, ammo availability (particularly in the GTA stores) and cost are usually about the same for both 12 and 20. However, the price, regardless of gauge, 12 or 20, does go up in the case of a premium product.
Price of saboted slugs (meant for use in a rifled barrel) either a 12 or 20 will be much higher compared to slugs designed for use in a smoothbore.