177 or 22? That - a swift peek at any of the popular airgun forums will quickly assure you - is most certainly the question.
To cut to the chase, the answer's 177.
Why? Well: it seems to boil down to trajectory - the parabolic path that a hopefully rabbit-bound pellet describes after it leaves the barrel of a rifle. It does not go in a straight line, I learned to my surprise, and then drop when it's tired - it inscribes, instead, an arc 'twixt gun and rabbit. But if it does that, then it's not simply a question of 'point and shoot' at all; distance becomes a crucial factor. How can you predict where in its arc your pellet is going to be when it reaches the head of the rabbit that you wish to shoot? Because if it's an inch too high then it will miss, and if it's an inch too low, then it will miss again. Nightmare!
177 pellets have a much flatter trajectory in the 40-odd yards after they leave the rifle barrel than 22's do. It's as simple as that. You can arrange it - I gather - so that in the area, say, between 20 and 35 yards (the crucial area within which the vast majority of relatively humane air rifle shooting is going to be taking place) the pellet will deviate from an imaginary straight line drawn between the end of the barrel and the rabbit's brain by a matter of only so little as half an inch or so. A 22 pellet, on the other hand, will be drawing a line that looks like the path of an amusing roller-coaster ride.
This I learned after I blew all the cash I had to spare on a 22 rifle.
If you want to have a gander at some parabolas, head over to http://www.chairgun.com/ or to the ballistic reticle calculator (superb name!) at http://www.hawkeoptics.com/brc/index.php and download the free software.
[In my time machine I revisit this post to leave a link to the future where I will write more upon this topic and my decision to opt for .177: http://austeritygrub.blogspot.com/2010/03/why-177-for-hunting-rabbits.html ]