This debate has been going on for decades, and there's still no definate answer. As Lurcherman said, they used to say .22 = fur & .177 = feather, but i think it comes down to personal prefrence. Some people swear by .22, because of the greater punch, some people prefer .177 because of the accuracy and penetration. I suppose it all depends on how you shoot, what range you shoot at etc. I've always used .22 because it's what i've been brought up with, all my dad's airguns were .22.
I'm sorely tempted to get a .177 next time, but for me it will be a leap of faith, because i've never used one