We are looking to spend around a month in NZ in the summer of 2011 but with such limited time, we are not sure we will have enough time to make the most of both of the islands... so sticking to one of them might be better!
We have heard the pros and cons of both but still undecided!!
Any advice would be amazing!
If you want to just do one or the other I would choose the South Island, but with a month you should be able to see many of the highlights of both. I'd spend 7-10 days in the North Island and the rest in the South. I presume you will be flying into Auckland so I'd recommend spending a day there, then heading on to Rotorua for a couple of days. From there you could go on to Taupo and then Wellington where you catch the ferry over to Picton on the South Island. From Picton you could travel on to Nelson which is a nice place to spend a day or 2, and then on to Abel Tasman which is beautiful and great for walking and kayaking. After that I'd recommend travelling down the West Coast and taking in the glaciers, then carrying on south and taking in Queenstown, Te Anau and Doubtful Sound and/or Milford Sound (I much preferred Doubtful Sound - prettier and much less touristy). You could finish your trip in Christchurch, travelling via Mount Cook and Lake Tekapo.
If you are into walking, I highly recommend doing the Tongariro crossing which is a fantastic walk; you can base yourself in Taupo or Turangi for this.
If you are going to be travelling in the NZ summer make sure you book accommodation ahead, especially if you are going during the school holidays (December - January).
Hope this helps you decide!
I was in the same situation in 2006 - one month only, and decided to do just one island instead of both. Decided nature and scenery is what I'm after so I went down south.
You could do a little figure-of-eight tour:
Christchurch --> Kaikoura --> Nelson --> Westport --> Punakaiki --> Greymouth --> Christchurch --> Dunedin --> Wanaka --> Queenstown --> Milford Sound --> Fox/Franz Josef Glacier(s) --> Greymouth --> Christchurch
From Christchurch (at the end of the journey) you can then see how much time you've got, and decide if you want to do Lake Tekapo, Akaroa, and Hanmer Springs (in order of my preferences!) using Christchurch as a base.
It does depend on how much you want to see and how much time you spend behind the wheel. Each island is about the same size as England. For me, I would have no trouble seeing both islands in one month.
It does seem that most people go for the south island but I think the north has a lot to offer too (though not the scenery).
You travel between the islands by leaving your car at one side, getting a ferry across and they will have a car waiting for you on the other side.