Check out both schools and the IB curriculum if you are interested in switching over to a different education system altogether. IB is certainly a more rigorous curriculum than NCEA with its compulsory units and its requirements like service and tok, an additional language etc. But if you are someone who has discipline, a self-starter and independent learner and wants a more holistic approach to learning, then IB can suit you very well.
The main difference between the schools you mention is the fees and location. Kristin excels because it is a complete school from primary to senior, and hence has the whole campus school experience where they can provide a lot of facilities for all its students.
ACG is a much smaller school, in a central city location (off Queen Street) and so is constraint by space and size, lack of facilities. ACG classes are much smaller, so there is more individual attention from teachers. And of course no uniforms in ACG, so it feels more like a uni than a college and so the students are treated more like young adults than older kids in uniforms.
However the school doesn't emphasis on school spirit (like Kristin) so if you are someone who is not interested in extra curricular activities, there is no push for you to join in something you don't want to. Neither does ACG emphasis on sports like Kristin has on swimming and team sports. But if you are an self-starter, and you want to improve and expand your interests, you can join in the student activities, hold leadership positions or even start one activity yourself. I suppose it is about setting out goals and finding out boundaries for yourself.
I would suggest checking out both schools open days and ask all the relevant questions you want and need to ask about making a decision. The next 2 years of your life is quite critical. You need to know if you have what it takes to catch up to the IB curriculum so you need to know where the schools are at and if you can make a smooth transition from NCEA to IB.