If you must shoot a cat, do not use an airgun. It's not powerful enough to kill a cat humanely. Use a 22 rimfire (handgun or rifle is fine), and if noise is a problem use a .22 CB cap, which will make less noise then an airgun. I still don't recommend shooting the cats, but if you absolutely made up your mind to do so, use something more powerful then an airgun.
As for the original topic, I think airguns are fantastic. I bought one years ago and have put thousands and thousands of pellets through it and it really is a fun way to plink and practice marksmanship without the noise and danger of a firearm (although there still is some danger). The one linked here I think is marketed toward youngsters, keep that in mind if you order it. For an adult there are better choices (though more expensive).
