I ran through this in a comment elsewhere already. However, I joke woke up in the middle of the night/morning (almost 5 AM here), so I might skip something in here. If you're using an AT&T individual plan, you can get 450 minutes for $40/month. Texting is $10/month. Data starts at $20/month. For me, I can live on those things VERY easily, as could most people. The 450 minutes includes free mobile-to-mobile, so you're unlikely to break that 450, unless you're a frequent caller of landlines. Personally, I can't think of any time I've topped 100 minutes on AT&T, let alone 450. I already operate on having my cellular data off on my Lumia 920. It's easy to pin a tile to the setting, and flip it on once or twice I week, when I actually need it. I probably use 50-100 MB in a month, as I'm almost ALWAYS in WiFi.
All of this leads to a $70/month bill. With that T-Mobile plan, the cheapest plan is, what, $75/month? As I might have mentioned before, I'm basing that on the (possibly-wrong) information Yahoo offered up. They said it was $90/month for unlimited everything. They ten said the lowest data plan was $5, and the largest was $20, meaning if $90 includes unlimited data, the lowest plan would be $15 than $90/month, or $75/month. In that scenario, assuming that Yahoo gave me the correct information, the T-Mobile plan is $5 more per month. You also get the Galaxy S III (it was he phone in their example, IDK what the others are priced at as a result) for $70, while a Lumia 920 would be $100 on AT&T. So, for an extra $30 for the device up-front, I'd save $5/month and get a device I preferred, plus a cellular service that gets better coverage in most places (or at least has a wider national coverage).
After that two years, the T-Mobile contract would drop to $55/month, but you might want a new phone by then. By the end of 2 years, the T-Mobile plan would cost $120 more than the AT&T one, while saving you $30 on a phone, meaning $90 more when all is said and done. This is the scenario I would operate under, though I'm not sure if it's a commonplace one--being more than capable of surviving with 450 minutes and 300 MB of data on AT&T or unlimited minutes and 200 MB of data on T-Mobile.
I just think that the cost difference will ultimately be negligible for most, unless you're a big-time talker outside of the mobile-to-mobile area, such as with a business. Even then, I'd rather pay the maybe $5-10/month more on AT&T to get better coverage and superior phone choices. Correct me on any of that if I am wrong, just working on the knowledge I was given.
When did unlimited texts become $10 a month? It shows up as $20 for me on the AT&T site.
Lowest T-Mobile data plan is not $5. It's free. $50 a month for unlimited talk, unlimited text, and 500MB of data included. I think you're looking at the old T-Mobile plans since you mentioned 200MB of data.
Math:
T-Mobile:
Plan - $50/month * 24 months = $1200
Most High-End Phones - $99 up front + $20/month * 24 months = $579
Total - $1779
AT&T:
Plan - [$40 (450 min) + $20 (unlimited text) + $20 (300MB)] * 24 months = $1920
Phone - $99 for Lumia 920
Total - $2019
That's with the Lumia 920 at $99. If you want an iPhone 5, AT&T becomes $100 more expensive because the iPhone rarely sells for less than it's $199 MSRP. Exception is T-Mobile where it follows the $99 + $20 * 24 months pattern for a total of $579. Cheaper than Apple's full unlocked iPhone price of $649. Also, remember than T-Mobile is no contract just like the prepaid plans I mention below. However, if you finance your device you still have to pay off the remaining balance. After that, the phone will be yours and can be unlocked. If you'd like, you can also trade it back in to T-Mobile and that'll help reduce your remaining balance.
But if you're really interested in saving money there are better options, as long as you're willing to put up with some negatives like poor customer service or no roaming.
Prepaid Alternatives:
Net10 (AT&T network, might have roaming):
Plan - $50/month (unlimited talk, text, 1.5GB data) * 24 = $1200
Phone - Lumia 920 from AT&T $450, Unlocked iPhone 5 16GB $650
Total - $1650-$1850
Cons - Poor customer service.
Airvoice Wireless (AT&T network, no roaming):
Plan - $40/month (unlimited talk, text, 500MB data) * 24 = $960
Phone - Lumia 920 from AT&T $450, Unlocked iPhone 5 16GB $650
Total - $1410-$1610
Cons - No roaming
Straight Talk (T-Mobile network, might have roaming)
Plan - $45/month (unlimited talk, text, and data) * 24 = $1080
Phone - Unlocked iPhone 5 16GB $650 4G HSPA+ only in areas where network upgraded (1900Mhz refarmed)
Total - $1730
Cons - Poor customer service, possibly same customer service as Net10. Both are owned by Tracfone. T-Mobile coverage (con for some people)
T-Mobile prepaid (voice and text roaming, no data roaming)
Plan - $30/month (100 minutes, unlimited text, 5GB data, 2G speeds past 5GB) * 24 = $720
Phone - Unlocked iPhone 5 16GB $650 4G HSPA+ only in areas where network upgraded (1900Mhz refarmed)
Total - $1370
Cons - Some people report that T-Mobile prepaid customer service is outsourced, only voice and text roaming, not data. T-Mobile coverage.