Rimfire. defo. Like cbw says, it will be a fanny on re-filling it and checking the gas etc. The rimmy is ready to rock and roll always.
Cost too. With tighter gun laws air rifles ( even fac ones) are going up in price whereas bullet guns arent. A rapid 7 will cost you £700 say, where my cz style rf, a gun i will never part with as its great, cost only £300.
Plus all you need to do with a rimmie is feed it cci stingers to up the ante, or shorts to lessen the power if the need arises.