Here are links to the tariffs of the main UK networks. At first glance, if 3 provided unlimited data then it would be the one to go for. But it all comes down to what you want . . . longer contract, pay more for the handset, unlimited data.
Big thing for me is that I am now less interested in calls and texts and more interested in the web capability – but the tie-in costs for unlimited data deals are big. Better to buy the handset maybe? Errr, but I don't have the cash right now.
Prize for the worst tariff page goes to O2.
Here are the iphone tariff pages . . .
And here is the Carphone Warehouse comparison page