http://www.youtube.com/watch?v=QIjmWlTOSZY
Here's a speediest showing the Straight Talk T-Mobile 4G AWS NanoSIM in Culver City, CA on an iPhone 5S.
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AT&T Go is great but its far more expensive. You pay $15 more a month for less data, and each additional GB of data is $10. If you have an iPhone 5C or 5S and have good T-Mobile service in your area, its better to get the T-Mobile Straight Talk SIM because Straight Talk is not enforcing the new 2.5GB limit on T-Mobile yet (and most likely never will). You do not get T-Mobie LTE with a straight talk t-mobile SIM, but I find that their DC-HSPA+ network is often much better then AT&T's crappy and congested 4G LTE at least in LA.