As SpursMadDave have said , it depends where you live. Also, I have been with Vodaphone in the past and the signal at my home address used to be good. Not anymore. I am with T-Mobile and have to say that there signal is great where I live. 3 Network are fine until you have a problem....................hmmm, the less said about them the better.
When it comes to renewing, just get a SIM only deal and buy your phone outright if you can afford to. You then get a phone which will work on any network and you are not tied into any long contract, especially if you only sign up for a 30 day rolling one. I have saved a fortune by doing it this way.