Yeah definitely Apple. Has others have mentioned, if bought from Apple, they are unlocked, so you have total choice of what carrier you want to use.
I live in the UK, but I don't live near an Apple Store (nearest to me is 90 miles away), so I order my iPhone from Apple's website. In the UK, if there is a problem with your iPhone and like me don't live near an Apple Store and like me have needed to order from Apple's website, you have a number of choices if the iPhone developes a fault.
1: Take out an Apple Care agreement, when you contact Apple Care to request a repair. This will then enable you to a free pick up of the faulty one and delivery of a new one at the same time. If there is stock you have a new iPhone the very next day or the day after.
2: Pay I think it's around £30 for the same service as above but obviously it wouldn't give you the additional feature of the Apple Care cover plan. Basically you would just be paying for the pickup charge.
3: Apple will send you out an empty packaging, which you then place the faulty iPhone in, which is then returned to Apple, it will either be repaired or replaced. This option has a total turnaround of about 10 days.
Not sure if you live near an Apple Store, so I thought I would point out the options available if a repair is needed. I've dealt with Apple/Apple Care on a number of occasions and I always find their service is outstanding.