I don't have any .177 experience on game, but I whacked enough small stuff (up to rabbit size) in my misspent youth with a Crosman .22 pump to know it's a killer. For bunnies, there might not be much difference and .177 pellets are, as you doubtless know, a good bit cheaper.
One of the attractive features of the Air Force PCPs is that some models let you switch calibers.
