As the global race for lowest cost & highest profits makes all such long flights [almost] unbearable, there isn't much difference in seat comfort or service nowadays between airlines.
With how you presented this, I'd probably choose UA for comfort in E+, but I calculate your pricing slightly differently: NZ you say is ~$600 and UA is 40k miles? 40k miles to me is worth $800.
For this reason, I might choose NZ.
BUT also worth considering: booking with award miles on UA will likely give you better/cheaper change/refund options should your plans change.