I doubt you can return to either as if it does break, and you take it back, all they have to do is check if you have an O2 contract, if you don't then they'll know you have jailbreak it.
Why are you worrying anyway? Do you plan to break it in 14 days?
I've returned 4+ iPhones at 02 because I scratched them and wanted a refund (I told them it was an unwanted gift )
Basically, they take the box, check the IMEI number matches and give you a refund.
Carphone Warehouse do NOT accept returns on iPhones unless you've failed the iTunes tariff credit-check.. and they'll pass you on to Apple if it's faulty, they're a joke.
All 4 phones were fully unlocked when I returned them, I'd just done a restore first.