O2 coverage map and here for Vodafone's network map - that show areas of 3G coverage. If you don't have any coverage where you're likely to be, there's not a lot of point getting an iPhone.
We tested by going to four locations, Waterloo station at rush hour, Millbank in Westminster, Oxford Circus at midday and in the evening Borehamwood, Hertfordshire. At each location we ran the free app Speedtest.net. This gave us a download and an upload speed in Kilobits/second. We ran the app at the same time on each phone, ran the test three times and then averaged the result.
At Waterloo station, Vodafone was comfortably ahead, with higher download and upload speeds.
At Westminster, it was a split result as Vodafone was ahead for download speeds, but behind O2 for uploads. Undoubtedly though, downloading speeds are going to be more significant, most of the time, so it's a moral victory for Vodafone.
Oxford Circus, proved to be a very tough test for both networks not surprising considering the number of iPhones likely to be in the vicinity. However, once again Vodafone came up trumps though, while O2 really struggled here - failing to even score on the first upload test.
Back at our home base though in Borehamwood, O2 clearly has better coverage, with better average speeds throughout.
