Looks like the government, through NZTA, has written to the Greater Wellington Regional Council recommending the adoption of Auckland's HOP card in place of Snapper - the HOP card being provided by New Zealand Transport Ticketing Limited, wholly owned by NZTA. Sounds like quite heavy handed tactics from NZTA and seems like a conflict of interest and potentially inappropriate conduct should GWRC go out to tender for an all-of Wellington transport card provider.
I don't know much about HOP, but it appears that Snapper has many added benefits - I particularly like Snapper on Semble and the ability to top up a physical card instantly via Snapper Feeder USB or my phone. I also like that I can use it for parking, etc. I guess, to be balanced, a disadvantage is that Snapper is not accepted by all bus companies in Wellington, probably because it's owned by Infratil, who are in competition with Mana, Newlands Coaches, etc.