iPhone 5 pre-orders have begun. The phone will reach consumer hands in a little under a week from now. If you are living in the USA and want an iPhone 5, there are three very important facts you need to know before you make the decision on which carrier to purchase the iPhone 5 from. Baffled but curious? Here, I will explain.

Global Roaming

The iPhone 4S introduced something new to the iPhone: global roaming. All iPhone 4S models, regardless of if they were on a CDMA network (Sprint, Verizon) or GSM network (AT&T), had that ability to pop in a SIM card and be used on any global GSM network (3G was not supported on all networks, however). The iPhone 5 carries forward this trend… with a twist.

As we all know, the iPhone 5 has 4G LTE. What we all don’t know is all iPhone 5s are not equal. The LTE frequencies used in Sprint’s and Verizon’s iPhone 5 are globally supported, meaning you can take your Spring or Verizon iPhone 5 outside the United States and get LTE connectivity on most LTE GSM networks. The AT&T iPhone 5, however, uses AT&T-specific LTE frequencies. This means AT&T iPhone 5 users will not be able to access LTE on most LTE GSM networks outside the United States.

[Note: All three Verizon, Sprint, and AT&T iPhone 5s can be used for global roaming. The difference is the first two can be used on LTE networks around the world while AT&T’s iPhone 5 will only get 2G or 3G.]

If global roaming is important to you, the winner here is obviously Sprint and Verizon. But hold your horses for a second; AT&T has something going in its favor, too.

Simultaneous Voice and Data

Sadly enough, the iPhone 5 on Verizon and Sprint does not have the ability to simultaneously do voice and data. This means if you are talking to someone, you cannot surf the Internet at the same time; or, you lose Internet connectivity whenever you receive a call. This is sad because Android phones, and other phones like BlackBerrys, have been doing simultaneous voice and data on Sprint and Verizon for a long time now.

On the other hand, AT&T’s iPhone 5 can do simultaneous voice and data. So while you won’t get global LTE with AT&T’s iPhone 5, you will get the ability to to talk and surf at the same time.

Unlimited Data Plans and FaceTime

The last point of contention you should keep in mind is data plans. As I’m sure most of you know by now, AT&T and Verizon have stopped selling unlimited data plans and have grandfathered everyone that had unlimited data plans. Verizon, however, is forcing anyone with a grandfathered unlimited data plan to “upgrade” to a data-capped — or family shared — data plan if they want to purchase an iPhone 5 at the two-year contract subsidized rate. If you refuse to move to a data-capped data plan with Verizon, you need to shell out $650/$750/$850 for a Verizon iPhone 5 (depending on which iPhone 5 you want).

AT&T is allowing customers to keep their grandfathered unlimited data plans and still purchased an iPhone 5 at the subsidized price (when eligible, of course). However, AT&T is going to charge extra for anyone with a grandfathered unlimited data plan that wants to use FaceTime over 3G/4G. Verizon is not charging extra for use of FaceTime over 3G/4G.

Sprint, on the other hand, offers the best of both worlds: unlimited data and FaceTime usage over 3G/4G without extra charge.

[Note: Keep in mind at this time Verizon has the largest LTE network in the United States, followed by AT&T, then Sprint.]

Conclusion

I’m sure picking which carrier to get your iPhone 5 on will likely be influenced by whom you already have. However, if you do have a choice of moving carriers, you need to ask yourself what you value more — global roaming, simultaneous data and voice, or unlimited data plans/FaceTime — and pick the carrier accordingly.

[Thanks Khalid for tip on simultaneous voice and data.]