It took me a whole day of running around on an empty stomach but I've completed a test of XT vs. Vodafone 3G speeds in Dunedin on Friday 7 August 2009.
http://buttonmasher.co.nz/ahmad/2009/08/10/xt-faster-in-more-dunedin-places-for-iphone-3g/
It was performed around various Dunedin landmarks, and the results as published show XT ahead of Vodafone for speeds using an iPhone 3G.
I have been since informed that there may be problems with the validity of this data because apparently there is a known problem with the iPhone when used on the Vodafone network, but in my defence I have asked for clarification before and not received any answer.
Therefore I went ahead with earlier plans to do this speed test, and the test was done with the information available to me as a lay telecommunications user.
Comments welcome.