Pick what represents a good deal to you, that's nice, fits well, and shoots well. Also consider what ammo your local RFD stocks, or if reloading, what bullets you would like to have access to, etc.
I personally picked .308 because I felt it would complement my .243 nicely, it's not too hungry for powder, and bullets and reloading information is available a plenty. Also, brass is about half the price of other calibres. You'll find more ballistic data on .308 then any other calibre, IMHO. I also felt that if I am travelling to different parts of the UK for stalking trips, and need to top up on ammo from a local RDF, there is a much better chance of getting .308 than some other more exotic calibres.
just my opinion of course