For the Galaxy S III, the total, two-year cost is $2270 on T-Mobile. By comparison, Sprint's two-year cost is $2840. That's $110 per month for an unlimited talk, text and data plan, plus $200 for the phone. Once again, T-Mobile wins. The only way Sprint comes out ahead is if you get a plan with just 450 voice minutes for $80 per month, which brings the two-year cost down to $2120.
It's also worth noting that T-Mobile's hotspot options are more favorable for unlimited plans. On T-Mobile, you get 500MB of mobile hotspot usage with an unlimited data plan, compared to nothing on Sprint. A 2GB hotspot plan from T-Mobile costs $10 per month, compared to $20 per month on Sprint.
The bottom line
Pricing aside, T-Mobile has some drawbacks. Currently, you can't get an iPhone through T-Mobile, although this will change. T-Mobile's nationwide coverage generally isn't as strong as AT&T and Verizon, and T-Mobile is behind in rolling out its 4G LTE network. While T-Mobile is expected to turn on its LTE network this week, it'll be some time before the carrier is fully stocked with 4G LTE phones. Many metropolitan areas are covered by T-Mobile's speedy HSPA+ cellular network however, which is promoted as 4G but might be more accurately called 3.5G.
Strictly in terms of cost, your dollar will go further with T-Mobile, but the biggest benefits are for families, for customers who need unlimited talk and text, and for users who plan to keep their phones for a lot longer than two years. If you're okay with fewer minutes, less data or no included text messages, you may still save money in the long run by going with another major carrier instead. Do your homework!
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