I would go for a spring powered one as the co2 powered ones I have never been quite as powerful, also if you are going to be shooting pest i would go for a .22 rifle and use pointed pellets not round nosed ones.
But for target practise dome head or flat head pellts can be fun, or if you can get hold of them hollew tipped pellets or the ones with a plastic spike on the tip are really nice to shoot with.