Upgrading my mobile phone from HTC Desire to iPhone 4S

I'm just about to become eligible to upgrade my mobile phone contract. I currently have an Android HTC Desire on a Vodafone contract, which has cost me £31.64 on average over the last three months. It was initially only supposed to cost me £17.92 a month, but I made the schoolboy error of buying it through Quidco and Mobiles.co.uk with (I should have known) too-good-to-be-true cashback offers. [tl;dr]