Definitely base yourself in the South Island, nothing really to see in the north island. Flying out of Wanaka would be the best choice, closest flight school to the best parts of the southern alps, however accommodation is expensive as this is a small tourist town. Christchurch is the largest city in the south island with plenty of choices for night life and accommodation, the local company Garden City Helicopters is based at Christchurch international airport and offer scenic flights or training in AS350, R44 and R22, great training area and very close to some very scenic locations. I did my flight training in both islands and there is no comparison.
