I've seen some threads on GZ suggesting Spark's coverage in Ohakune was more robust than Vodafone (which may have been overloaded?), but these were a couple of years old and may well have changed.
Anyone with recent experience that they'd care to share? My cell is on Vodafone, so I'm thinking about hedging my bets and going with Spark for the data stick and falling back to tethering if I have to.