You need to compare network features too
Three have a content lock on internet access, you have to pay extra every month to override it!
Tesco looks the best bet especially with 12 month contracts but they don't support call forwarding so Hullomail will not work on their network. Hullomail is required if you want visual voicemail functionality and O2 are the only network to implement visual voicemail.
Tesco and Three also both get a bad rap for customer services especially if you end up with an overseas CS rep, good luck if you deviate from their scripts!