For MexicoWanderer,
A Galaxy S2 if AT&T is 4G or pseudo 4G in speed. It uses HSPA+ for it's 4G or something like that service. It's chip, the Snap Dragon 1.2 Gh does not have 4G LTE capability, like Verizon offers. Verizon never carried this model.
If you get one, go to the PlayStore Icon App on one of the screens by Google (it'w where you can go to get all the free apps, for free, and some for pay, if you want to go that way) and type in "OOKLA" for a speed test app. You can run the app for free and it will tell you if your upload or download speed is good, so so, or bad.
AT&T will generally choke down your speed when or if you get close to your data limit.
Go to "High on Android" and do a search for "Galaxy S2 AT&T how to root JellyBean 4.1.1 (Jelly Bean is the most current up to date operating system) for AT&T. You may need drivers from Kies Samsung to do so, on your desktop, first. Once your root your phone, you have super moderator powers and can shut down AT&T cr*p bloatware running in the backround.
If you are on limited data plans, turn your data off when you are near a wifi point, and try to use free wifi instead, always. If you are not near a wifi point, turn your wifi off, and turn data on , and use what you have for data carefully. Wifi is usually free, like at Mcdonalds, where you can get free coffee in the mornings or for very cheap, $.59 and free refills. Data and using it on your phone, is on your dime.
Do a search on Google "How to use a Galaxy S2 with android" to learn how to operate your new phone. I think I threw in a protective case with the phone, a BodyGlove model. It only works or fits with the smaller 1700ah stock battery. There's a fatty battery in there to, with a back, 3000mah, lasts much longer.
You will also have to do a search on how to unlock your S2 for free on the internet. You can then probably install another brand SIM card in your S2 down there in Mexico, and it will work fine. Or it should.
You have a lot of reading and studying to do to learn how to get the most out of a smart phone with an Android operating system. Trust me, it is worth it, there are brilliant hackers out there that make open source Android systems the "go to" system for the intelligent smart phone users.
A good starting point. In it's day, a few years back, it was the best of the best.
http://en.wikipedia.org/wiki/Samsung_Galaxy_S_II