Should I go with vodaphone contract?

I just got a phone call from vodaphone offering me samsung galaxy for a two year contract. Right now I am with O2 top up and I pay 15 pounds a month and have unlimited texts and calls to O2. That's enough for me. They offered me contract for 10 pounds a month for a year and then the second year for 16 pounds a month with 300 free minutes and unlimited texts to any phone. What should I do? I have an old phone but I don't want to find myself in a situation where I have to pay a lot... I'm scared that with the contract that might happen. Right now I live with my bf and near friends but summer Im moving to london to study and Im sure I'll be using my phone at least abit mor eot call him and others. What would you choose? What is better for me? please answer quick becuase they have extra offer today and otherwise it would be much more expensive for me...