I am NZ based, and decided to go for *G with BMI, if you are flying paid C with NZ, you will get to gold very quickly. You won't be able to upgrade with NZ, but you can book cash&miles in C with them. The BD *G will also get you in to lounges in the US, but no E+ on UA.
Another good point about BMI in NZ is that if you earn miles on internal flights (cheap ones earn none) you will get 600 miles per sector, so by way of example, UA gives me 93 miles from AKL-WRE, BMI gives me 600.
Might be worth taking a look in the BMI forum for more details.
tht