Loach: on a general note, if I were doing my PPL(H) again I would definitely seek out an older more experienced instructor. I did my training at High Wycombe and although neither were my instructor, had the pleasure of flying once or twice with Mike Wilds and Alex Hoff. An hour with one of those two gentlemen was worth at least two with an 'ordinary' instructor.
On an NZ note, the two Christchurch based operators that you mention both operate out of the main airport. I don't know too much about how they compare but if you need more info I will try and help.