I'll skip to the answer - we went with Spark (a $19 pay as you go sim with 300 NZ minutes, 300 NZ texts and 500Mb data). If you want a little more detail - I'll fill you in on our experiences with the networks so far.
... like, a lot. I'm a Google maps navigator, reddit, twitter and news reader and get a bit twitchy when I'm away from emails for more than a day or so. You'll find that this may taint my view somewhat of how I view the various data plans, but I would venture that most travel types are really only looking for data with a few bonus minutes for calling and booking places while in NZ - Whatsapp, Skype or Google Hangouts look after the long-distance stuff.
To my despair, New Zealand's data options are, like most UK networks (at time of writing) quite stingy on the data packs.
The one thing that splits Spark off from the other networks is the availability of 'Spark boxes' - modified phone boxes in major towns which act as local free hotspots for Spark customers with a 1Gb/day allowance - much more attractive than the standard 500Mb/month. I will warn you though - they can sometimes be a bit dodgy and you will spend a little bit too much time camped creepily beside them in your car.
Sweet sweet internets!
... and there are plenty more scathing insults where that came from.
2degrees - after I'd burnt through our first Spark sim's 500Mb/$19 of data I thought it would be worth trying out your 750Mb/$20 sim (as we now had access to the Spark boxes) but you are terrible - we can't seem to get reception on any of the main roads down the West Coast of South Island (so far) while our spent, Spark sim seems to have connection even though we have run out of our allowance. This irritation is only exacerbated by your claims of 97% coverage. Not happy 2degrees, not happy at all.
Just in case you are in a rush - we normally can get back to you in less than a day!