Pro iPhone users, advise me (for a Bday gift)...

Hey,

So... I have just a few hours left to make a decision and I need your expertise.

Here's the scenario:

I'd like to get my wife an iPhone that we can use internationally, specific to our situation. And I need to decide which way to go, vis a vis iPhone choices. NOTE: it has to fit our travel situation, which is that we spend a LOT of the year in Europe and the same or less time in the States. I'd like an iPhone setup that lets us move inexpensively back and forth.

This means that it's a given we'll want GSM service, both in the US and Europe. It also means we do NOT want subscription service -- only pay as you go -- when Stateside. That might also be the case while in Europe, but where more open to a subscription on that side of the equation.

So now, here's what I'm wondering:

a) Do we get a subsidized contract iPhone in Europe and then unlock it ourselves, to use Stateside with AT&T pay-as-you go service? (I understand AT&T is the only pay-as-you-go GSM option, but correct me if I'm wrong about that too)?

b) Do we buy an unlocked iPhone in Europe -- paying the high price at purchase for that feature -- and then work out the service options later (hinges on whether that's possible, to have a subscription service on an unlocked phone)?

c) Do we buy an unlocked iPhone in the U.S., where the price for unlocked might be a little lower, and then also -- same as above -- work out the service options later?

I guess what I'm asking is, we ultimately want an unlocked phone, but what's the cheapest AND most reliable way to do that? Can I unlock a subscriber-based EU iPhone and then still use it for a subscription service, if we decide to do that? Or is it safer to buy a legit unlocked phone from the start?

EXTRA Question: Is it worth it to buy the iPhone 5 or is it worth the savings to just get the iPhone 4s?