How much data do you use?
Which provider are you currently with?
How much do you value the latest handset?
How much a month did you pay?
When you weigh up paying for a contract, you need to look at the whole price: handset cost (if any) plus the monthly cost times the number of months the contract runs for.
Contracts are ridiculously overpriced, even taking into account the handset. As an example, when I was choosing, I wanted a Samsung Galaxy Mega I9250. I looked at a contract with Three, and when I looked at the total cost for 24 months, I could buy a handset SIM-free from Amazon, plus leave enough money every month for pay as you go top-ups, PLUS buy a new handset at the end of the 24 months if I wished.
If you are using a huge amount of data, Three is a good choice, as you can either buy a SIM only option with unlimited data or buy a PAYG add-on that gives you 300 minutes, 3000 texts and unlimited data for £15 a month. Standard PAYG rates are the cheapest in the business too, at 3p per minute, 2p per text and 1p per Mb data.
As with any mobile contract or SIM only deal, beware additional costs over and above your monthly allowances. Three and Tesco Mobile offer capping facilities where you can set your limits.
If you are considering leaving for a different network, the best thing to do to check the coverage in your home and the areas you frequent is to buy or get (often free) a PAYG SIM card on the chosen network, load it with a fiver, pop it in an old unlocked handset and see how it performs.
Do not forget also if you want to move network you do not have to lose your number. You can port it to the new network. Ask your existing network for a PAC (Port Authorisation Code). You then give this to the new network along with the serial number of the new SIM card and within a couple of days (usually) the transfer is complete.
Do not forget to use up your balance or allowances before your number is transferred or you will lose them.