I suspect most people's decision is based on much more random factors than weighing up the pros and cons of each country. i.e. many of us came to NZ because we came here on holiday once and liked it, or we have relatives or friends here, or we saw a program on TV about it, or we just came on a whim etc. The same will apply to people who ended up in Australia. For some reason the rational approach of comparing the countries that you could emigrate to leaves me cold and seems somewhat mercenary (maybe that's just me though). I think emigrating works best when you can't really articulate why you came!