The North Island is New Zealand's indigenous cultural heart, and it's the best place to see and understand the history of the country. Like most Pacific/Polynesian nations, New Zealand has no written history: it's recited orally in song and story, The trouble with the North Island is how utterly fabulously popular it is with tour companies, buses, helicopters, groups large and small and just about every nation on earth walking around!
South Island wins every time for scenery, but loses (badly!) for the often atrocious weather, particularly Tiri o te Moana (Southern Alps), and the spectacularly beautiful (if moody and always changeable) MIlford Sound area. The further down the south you go, the worse the weather can be. In The Catlins on the SE coast in April 2015, our camper was battered by 160km/h winds and rain coming directly off the shore 6 metres away. Likewise, we were stuck in the campervan at Milford Sound for a full 2 days with cabin fever as a vicious southerly outbreak made its way through (part of camper's front roof cowling was torn off here, creating a leak). On the third day... brilliant sunshine! Just pack all your wet weather gear and be prepared for four seasons in one day in whatever time of year you visit!
If you like wine, the Marlborough region of the South Isleexports its wares to every corner of the world (including Russia and Antarctica).