I recently flew NZ business from SFO to AKL and AC business back from AKL to YVR at the end of at the end of 2023 beginning of 2024. So I’ll provide as much insight as I can there are others here who frequent this route much more so they will probably have more robust info but this was my experience.

First off you are right that only AC paid business class has access to the signature suite so if you wanted to eat on the ground pre flight to maximize sleep (or get two full meals/a better meal than on the plane) you will get a much better experience with AC. If you took NZ through the states you’d get access to the Polaris lounge but in my opinion the signature suite is a significantly better dining experience (only been to the SFO one but I have also heard this from others) so if dining on the ground matters to you in my opinion AC is a clear choice especially if getting Polaris meant a connection you wouldn’t otherwise have to do.

As for the seat overall I prefer the AC seat especially in the case of a day flight however since you will be taking an overnight I find that NZ edges AC out slightly when it comes to being a full bed thanks to the properly thick mattress pad they provide and the unrestricted footwell (since it is open to the aisle rather than a footwell that goes under the seat in front of you). However the NZ seat has to be fully converted as a bed and can’t be a proper lounger/recliner through the full range of recline and is much less private than the AC seats additionally if you are in the bulkhead on AC your footwell has more space as the space behind the IFE is open at the top rather than closed due to the seat in front of you. It will somewhat depend what is important to you (my mum favours NZ despite its drawbacks for the proper mattress pad and openness when it is a bed) but I still prefer AC especially if I have the bulkhead (1A or 1K).

As for the onboard service my AKL-YVR on AC was very mediocre certainly edible but nothing to write home about. My NZ SFO-AKL on the other hand was pretty good and was better than what I received on AC (especially breakfast NZ had four options to compared to two on AC (the notorious parsley omelette or waffles which I usually skip) and one of them was eggs Benedict where they poached the egg on the plane and it actually turned out quite nice). However neither provided food that was as good as the signature suite so if you have the time to eat on the ground and would just try some of or skip dinner on the plane AC wins out. If you are planning to eat on the plane from my limited NZ experience their food between NA and NZ is definitely better than that of AC.

Overall I wouldn’t say you’d be disappointed with either at the end of the day it’s business class and I was able to sleep on both of them quite well however for the signature suite and better seat (by my preference if you find the standard reverse herringbone seat without a thick mattress pad hard to sleep in you might prefer NZ if you are ok with its other draw backs) I would personally go with AC if they are a similar price. This is especially true if you credit to aeroplan as you collect SQD and LQM flying on AC which you don’t flying on NZ but it seems you are a UA flyer so this point doesn’t really matter. If there is a significant difference in price I would probably just go with the cheaper of the two since they were both pretty good on the route.