Air NZ have some of the newest 737-300s in the world, and they are very well maintained. I'm sure there are plenty of airlines ready to snap them up.
While I am a Boeing fan, as a passenger, I'm quite happy on their decision. Despite loving the 737 series, the seats are really crampt from about row 12 back, and I hope now Air NZ is getting aircraft that can carry more passengers, that they may leave more room between seats! From the passenger seat, as Tom said above, the only difference to me between the two is the different engine sound.

Thankfully Pac Blue are still flying 737s around, and there are a few on the international routes, so we've not lost 737s just yet!