Kia ora!
Definitely ATT. 3g works really well and they have been really easy for me to deal with. It is a bit expensive though so be sure to get the right plan. I have the tethered PDA plan so I have mobile broadband that lets me use my
n95 with my iPod touch or laptop. They seem to have the best selection of phones and aren't jerks when you ask about how the plan works with a non-carrier specific phone. They are not perfect though so don't expect that. You may have to hang up and call back sometimes, but for the most part I haven't had to deal with customer service for over a year.
You get mobile to mobile calling and a lot of people here have ATY now with the iPhone craze so free calling galore. Plus voip and all of that works fine too so you don't need a big plan.
I think that the only BIG major difference is 3g and Dayene is totally right about the battery drain. I don't notice of when I am in 3g range.