If you look at the top of the forum it explains all about SGC (shotgun certificates) and FAC (firearms certificate)
It's self explanatory, for a shotgun you need a shotgun certificate, which is the easier of the two to get approved. If you go for the rifle you will need the firearms certificate.
As you've worked out for yourself, what is "best" is purely a matter of opinion, taking into account the type of shooting you want to do. So a rifle would be pretty useless against birds in flight, clays and bolted rabbits, but try shooting a 75 yard rabbit with a shotgun and you'll find it sim