The argument over .177,.20,.22 blah blah is a pretty pointless one. Its a moot point. All these calibres are MORE than enough for anything from a rabbit down to a woodie. MORE than enough.
I personally always opt for .177, and though i own a PCP (AirArms S410), i've just purchased a AA TX200 mkIII , again in .177 as i wanted the pure self sufficiency a springer brings with it. The .177, in the right hands will kill just as quickly and as cleanly as any .22 out there. A recent study in Airgun World was conducted where pellets of varying calibres and weights were fired into firm, flesh consistency terracotta wax at 25-35 yards and the resulting channels filled with liquid plaster. Once the plaster had set the resulting cores were taken out for inspection. The .177 , lighter pellets produced much wider channels than the .22 pellets and also these plaster cores weighed upto double that of the cores produced by .22 pellets. This, according to the studies, indicated that the .177 rounds decelerated more on impact and also while travelling through the quarry dumping more energy as a result.
The same can be said of bullets. Heavier bullets designed for an animal such as a cape buffallo would be heavy so as to ensure they would keep driving through the animals heart and lungs even if they strike the bones of these large heavy animals. Rounds designed for killing foxes however are lighter as a fox only has thin bones, a heavy round would pass through taking much of its energy with it, the lighter round carrys less peetration WEIGHT and so deccelarates alot quicker than a comparitive .22 pellet so therefore the .177 is dumping a lot more energy as a result.
Thus far, i've never had a runner while using .177, over 15 years of shooting rabbit, woodies and corvids with it. .177 also suffers less from wind deflection and has better Ballistic Coefficiency/accuracy according to that study.
As this little youtube video shows, the only movement from this .177 shot rabbit is simply it rolling down the slope created by this tree's roots, it is still, however, stone dead thanks to the .177 pellet hitting it in the skull. Enjoy.
http://www.youtube.com/watch?v=KbKHxgdD3ow