Wow! You probably couldn’t choose two more different locations if you tried! I have been to both - New Zealand one time, on a tour (2024) and China seven times, both volunteering and touring (2010-2014, 2019, 2025). I loved New Zealand and would be happy to return and, obviously, I’m fond of China. Here are some thoughts.
New Zealand is a remarkable place and easy for an English-speaking tourist. The people are friendly and, although there are a lot of similarities to the US, they definitely have a culture of their own and a fascinating history. If you are looking for nature, New Zealand is hard to beat - coastline, mountains, fjords, farms, wildlife (including penguins) - they have just about everything. If you are looking for a tour, you might consider Mondumo Tours, operated by Colin Mairs, a Rick Steves guide. Website: https://mondumo.com/ This is the company I used and the trip was excellent.
China is, of course, huge, so if you only have about two weeks, you would need to limit yourself to one area of the country or take a really efficient tour. Gate One (https://www.gate1travel.com/asia-pacific/china-japan) has a 15-day small group trip that seems to cover the highlights and has a very reasonable cost (I’ve traveled with them twice but not to China. I recommend sticking with their small group “Discovery” tours with a limit of 22 people). China is certainly a more challenging destination than New Zealand - culture, language, history, food - but I find it so worth the effort!
It really boils down to what you want from a trip, what’s most important to you, and what kind of challenge you are up for. I don’t think you can go wrong.