Ahhh, but true versatility is a .20!
Yep! Right up until you try to find pellets for it!
Great in theory, crap in reality, unless you can afford to shop mail order when you run out of ammo.
Hard to find the Sheridan trash cans, let alone any decent selection of .20 cal, most places.
.177 is probably the best bet for the OP. Lots of ammo around, even in the boonies. A .22 would be my choice, on the energy front (same velocity, bigger slug= more energy) but if whacking the odd garden pest is not on the schedule..
Either would do just fine though.
I'd suggest staying away from CO2 guns, though. Consumables costs become an issue after a while, and the added complexity of seals etc., makes for more possible downtime. Springer would be first choice, Pneumatic, either muti or single stroke, as second, IMO.
There are a lot of things that can be done to smooth out the commercial products, if you are inclined to tinker. You can take a cheap twangy springer and make it into a gun that fires with a nice solid "thwunk" noise, with a bit of effort, most of the time. Lots of info online about modding spring guns to make them more civilized.
Cheers
Trev