Let's say I can't get my phone unlocked. I have an iPhone 5 (AT&T), so I think at this time it would be difficult to unlock, since it was a 2 year contract... Plus, I think prepaid nano sims are a lot harder to come by.

Carrying an extra gadget isn't that big of a deal for me, so size really doesn't matter. I'm just looking for something with decent battery life and obviously broad international use.