Which network is best for all round coverage? Telecom XT, 2degrees, Vodafone, or another?

I need to be able to upload data from my tablet. In particular,  Onehunga and Central Auckland/CBD area..

I'm using a Galaxy Tab PT1000. It's a T-Mobile one from the states I believe. it has a black back (the PT1000's often have a white back.. I dont know if this has any significance over determining its capabilities). It seemed to have issues with its data connectivity when I first tried it in Penrose, but sprang to life when I moved away from that area.

Any ideas would be appreciated. Thanks!