openmedia: My own tipping point is simply coverage. At home I get no 3G coverage on Vodafone or 2degrees. In fact I barely get any coverage inside my house on either of these networks. I'm not that remote, on the Glen Eden side of Titirangi. A couple of streets away I get perfect 3G coverage.
Then you're better off with 2degrees -- at least you'd be on EDGE instead of GPRS. It's not great but it's a noticeable improvement on GPRS. And great for my aging HTC Dream which doesn't do the extended range 850/900mhz 3G that vodafone and telecom offer.