Having been a top-tier FF in both programs for several years (AirNZ now primarily due to banked years, as I've switched over to Virgin and Qantas) here are the key points that swing in for Velocity for me:

- its much more generous program in terms of points and status credits. AirNZ are stingy by comparison - but then they can get away with it as they have a capitve market in NZ and a monopoly on several key routes, whereas Virgin have Qantas. Competition is a great thing for consumers.
- family pooling is fantastic - didn't realise how good until I started using it
- while Velocity has a much smaller partner network, if it works for you the quality is higher than Star Gold - First class lounge access in LAX and Abu Dhabi, Virgin Clubhouses, Virgin America and Delta is the US, Etihad lounges. Europe is a gap.
- redemption options are good - taxes are low (particularily against Qantas) and much better value than AirNZs dollar system. Singapore and Etihad are high quality options.
- onboard product in J is very good and soon to get better (although it lacks some of the polish of AirNZ sometimes)

It basically comes down to this - Virgin operates in a very competitive market, AirNZ - despite what they say - to a large part do not.