So you drop Southee fresh off taking a five wicket haul on debut against the same opposition? The bowling attack will Martin, Mills, Southee and Vettori. I have very little doubt about that. I was against Southee's selection when it happened but to drop him now would be absolutely ludicrous.
TBH I kinda wish Mason would play ahead of Martin, though I'm, obviously, aware that that won't happen. But yes, I do think it would be best for everyone concerned if Mason plays ahead of Southee right now. Southee struggled badly in his only game of the tour so far - hopefully he might play another, I'm a bit surprised he isn't playing this one. And if he does and does well then maybe I might reconsider.
But it's easy to lose sight of the fact that Southee, remarkably impressive though he was on debut, is still just 19. Mason is highly unlikely to end his career a better bowler than Southee, but I do think he's probably got the best chance of the two of success in an alien country (albeit not unfathomably alien) right now. I think it'd be best for New Zealand long-term and short-term for Mason to play ahead of Southee in the opening Test. Unless something compelling happens in the final tour-game. Not only do I rate Mason's chances better currently, but he has less than Southee to lose by bad performance now.
BTW, TBF the bowling-attack will be Martin, Mills, Oram, Southee and Vettori. The specialist-bowlers might exclude the all-rounder tho.
I'll be the first to admit that O'Brien looks absolutely horrible, and I was in fact against his recall in South Africa, but the man has been given six Tests on the back of being the standout bowler at domestic level for a few seasons running. Two of those were against Bangladesh. His selection was hardly unjustified.
Well, it wasn't the worst selection you'll ever see, no. But I do think I'd have preferred to see Mason picked in South Africa - presuming he was fit, and I can't remember for certain that he was.