If we're talking a complete swap of type, then the -42 must have a huge advantage - esp if talk of Mount Cook getting an ATR sim rings true. Better utilisation of an expensive asset, common type rating, spares, maintenance and they can do all the training here in chch... i like the sound of that. Folding Air Nelson into Mount Cook would probably be next on the agenda, but where that would leave pay & conditions who knows.
Also would we be talking a combination of lease/purchase for the -42's or are there quite a few used examples out there?