I feel like my question isn't being understood here.
I understand the difference between the two and if the sites were for example English and Spanish I would get which to use, but the hreflang tag sub-divides languages into individual regional variants, in my case en-GB, en-IE, en-AU and en-NZ.
So, my question is which to use when the "base" language is the same, so in all essence, the article is basically identical, with the odd regional spelling variation, e.g. colour and color, tossed in?