Calibre is very heavily debated. The majority of THL members seem to use .22 for hunting, however .177 is just as capable. It depends mostly on your quarry, .22 will comfortably kill most small game; crows, pigeons, rabbits, rats and so forth.
.22 is a lot easier to get hold of than .25, at least by my experience, I'm not sure about price though. I also haven't seen too many .25 rifles, that might narrow your choice somewhat.