If you get a .20 or .22 you won't have near the same ammo selection as .177.
Also - if you want to shoot competitively, it's all .177.
I have a .22 benjamin pistol, but I wish I would have gotten .177.
177 is cheap and plentiful, fine for pests up to rat size (or better with some of the newer guns), and is cheap and plentiful.
.22 if you are going to hunt with it, .177 for everything else. IMO.