Welcome to FlyerTalk!

In my experience it is fairly unusual to have a great experience travelling across the pacific in coach. It is fairly rough, but still it can be done - thousands do it every week. If you can sleep on planes it helps. I haven't flown Qantas to NZ but I know that the United flights are very packed. I would assume the Qantas ones are too.

I suppose it all depends on how you look at things. If you think of it as only being miserable for 12 hours (twice) and otherwise having a wonderful trip then I'd just go in coach. If you are happy to wait longer to build up the miles then that is a fine option too. You will definitely be more comfortable and have a more pleasant start to your vacation, but at the end of the day, unless you have any sort of condition or problem that makes uncomfortable seating extra difficult for you, you will survive!

In any case I would very much recommend visiting NZ in Feb or March as you said. This is the best time to visit as the temps are very nice. Unfortunately most Americans tend to visit during the summer here (school holidays) and end up down under when the weather is quite cold.

Best of luck deciding anyhow - you'll have a wonderful time no matter how you get there!