.22 is cheap and is the workhorse around the house for most of your killin in 'lower power guns' ..which is a virtue if you want to keep it confined to your own yard with minimal damage to the neighbors ... flower pots
.30 *ahem* well .30 is heavier hitting for the more robust pests, but my opinion is nothing a .35 wont do better for the same money .. may come down to the gun caliber availability, if'n say you want a special purpose gun to 'strip the paint' off something and its only available in .30, id go for it, otherwise id want .35 .. my take ..