I'm a Kiwi so I'm a little bias - strangely enough, I'm going to recommend NZ all the way! Sadly, I can't give you a break down of the South Island resorts as I've only ever boarded on Mt Ruapehu in the North Island but there are a number of ski fields down south so if you allowed enough time, you could do a 'tiki' tour around, see a lot of the island and hit quite a few of the resorts as well. Travelling in winter, you should expect a fair bit of rain - particularly on the west coast of the South Island. The scenery in the Sth Island is stunning - check out
Extreme Skier (world heli challenge), near Wanaka | Stock Photos of New Zealand, Africa, Australia and Fiji | davidwallphoto.com
You've probably already googled these but just in case, here's a few links to some of the fields in the South Island:
The Remarkables NZSki.com Home Page - Coronet Peak, The Remarkables and Mt Hutt
New Zealand Skiing, Treble Cone, Wanaka - New Zealand's Largest South Island Ski Resort
Mt Hutt NZSki.com Home Page - Coronet Peak, The Remarkables and Mt Hutt
Mackenzie Winter Marketing Group - Mackenzie District, New Zealand
www.nzski.com/coronet/
www.dobson.co.nz