T-Mobile rocks over AT&T plus I am out of contract. they treat me like GOLD.
and its $30.00 a month less for there services.
So you can have your AT&T
-VisaNick

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Now that you put it in
BOLD, I can totally understand your point.
Without a doubt, AT&T has better coverage. Maybe not in your particular area, but across the nation, AT&T blows away T-Mobile.
As far as voice plan pricing, here is what I found from both company's websites:
AT&T: 900 minutes @ $59.99;
T-Mobile: 1000 minutes @ $49.99
So, you may save $10 and have an extra 100 minutes with T-Mobile. However, with free carrier-to-carrier calls for both plans, the better deal may lean towards AT&T since they have a significantly higher customer base (which means more free calls). All of my friends have iPhones, so it is free to call them because they are on AT&T.
As far as data, I did not easily find that information on T-Mobile's website. I do know that I pay AT&T $20 per month for unlimited data and 200 text messages. Mind you, I have the original iPhone (not 3G). I am assuming that you have the original iPhone too because the 3G does not work properly with T-Mobile yet, right? (I have been out of the jailbreaking/unlocking seen for a while, so I am guessing that there is still no unlock for T-Mobile and 3G. Please correct me if I am wrong). So, what do you pay for data with T-Mobile? AT&T customers also have access to all of their WiFi hotpsots (most of which they stole away from T-Mobile when they struck a partnership with Starbucks).
So, to get from that $10 savings in voice plan to the $30 total monthly savings that you are referring to, I assume that T-Mobile gives you free data and free 200 text messages. Is that how you get the $30 in total savings?
I am not a salesperson for AT&T. I just don't get why T-Mobile is so much better (and keep in mind that I used to be a T-Mobile customer).
Most importantly, my AT&T iPhone works fine and yours doesn't. As far as their customer service, I am under contract and still treated like GOLD.