I'm currently with Vodafone but my contract expires at the end of the month and I am cancelling it.
I bought myself a Google Nexus 4 before Christmas. State of the art smartphone for £279 and I've got a GiffGaff SIM, £12.50 for 250 any network mins, unlimited texts and 1gb of Internet.
I don't think I'll ever go contract again.
Anyway, in answer to your question, I'm on my third Android phone and love them, as it stands I have no intention of getting an iPhone ever. My biggest problem is being tied in to iTunes. With my Android I can plug it in to any computer (even a Mac) with a standard USB cable, the phone appears like an external hard drive and i can drag and drop anything to or from the phone, music, photos, video. So much easier.
In my opinion the Galaxy SIII is probably the second best phone on the market (after the Nexus 4) but has been out a while and will soon be superseded by the S4.
So in short, get a Nexus 4!