iPhone 5S is £39.50pm on a two-year deal from Tesco with 2GB data

If you're looking for a free iPhone 5S on a contract, I reckon Tesco Mobile has the best value deal. The phone-flogging supermarket is selling the top of the range Apple mobile for free on a £39.50-per-month two-year contract, which also nets you 1,000 minutes, 5,000 texts and a mighty 2GB of data.

Stock will be available "in the coming weeks," Tesco says. Here's a handy chart with all of Tesco's options, including for the perky plastic iPhone 5C:

 

There are cheaper ways to bag a free 5S, but you don't get as much thrown in. Vodafone, for example, has a two-year deal for £38, but you only get 600 minutes and 500GB of data.

Three has a £37 per month deal, with 500 any-network minutes and unlimited data, but you have to splash out £99 upfront. That'll end up £40 more expensive over the length of the contract -- worth considering if you reckon you'll use much more than 2GB each month.

O2 splits its contracts into paying for the phone and paying for services. You can get unlimited calls and texts and 1GB of data for £17, then a free phone for £25 per month, totalling £42 per month.
Those are all 3G contracts, mind. If you want 4G data on your brand-new blower, you'll have to sign up with EE, Vodafone or O2 -- Three doesn't launch 4G until December. An iPhone 5S on a 4G contract with Vodafone starts at £47 every month, with 6GB of data, and you have to pay £79 upfront. Ouch.

EE isn't much better. For 10GB of 4G data per month and £50 upfront, you'll be shelling out £46 every month. You can see its other options here.
Tesco runs on O2's network, so its 4G availability will be the same, geographically, once it starts offering deals, which it says will be later this year. You won't have to extend your contract, but you'll almost certainly have to pay more.
All these deals are for the 16GB version of the iPhone 5S. If you want more memory, you'll have to pay more.

Apple is selling the 16GB 5S for £549 SIM-free. You'll have to wait a week or so before other dealers get their hands on SIM-free models, but they'll be the same price. By comparison, 24 months of Tesco's £39.50 deal will end up costing you £948 -- nearly double.
If you can, save money by buying it outright and get a cheap SIM-only contract for a tenner a month until Three starts its 4G service. Then you can properly compare the service you get in different areas, how much data you really get through, and it won't cost you much more than a bit of hassle moving your number across.