I've heard overall Orange have better 3G coverage but both are quite good. The tariffs are pretty much the same although O2 give you 500mb of data vs Oranges 750mb data limit. Also, you could factor in that O2 do deals like priority tickets to the O2 arena whilst Orange offers Orange Wednesdays and Magic Numbers. I've been on both networks, currently on Orange and when I check my balance online I've found both websites to be pretty p*** poor. Also, I've heard if you are a gold member on O2 you can upgrade your phone up to 6 months early, although I'm not sure if that is true, I think it costs extra as well. I'm certain however you are allowed up to upgrade your phone from 3 months before your contract ends on Orange. I hope this helps you make your choice.