In my opinion the contract prices on Three right now are horrific. It's £174 cheaper to go sim only one plan (£15 a month) and buy the phone at £549.99 compared against buying the phone on the oneplan from Three for £99.99 and £41 a month.
If we compare a £37 ultimate internet 500 Three plan (the cheapest one I can see) against the tmobile deal above the Three price is £200 more in total cost over the 2 years.
Previously Three contract pricing has been some of the most competitive on iPhones, is that 4g network really free? Seems to me that new contract customers are getting a raw deal.
No 4g is an obvious downside, but not needed by a lot of people.