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Depends on the coverage for each carrier on the areas you plan on using your iPhone 4. Also, if you go the T-Mobile route, you will not be able to use 3G data. AT&T is the only U.S. carrier to support the unlocked iPhone 4's 3G spectrum (GSM).
 
Signal wise, an iPhone will be capable of having the same signal strength on either network. What it comes down to is the service area provided by each network. If you are in an area that is completely covered by ATT and T-Mobile, you won't find a difference in strength. If you are in an area that has better coverage by one than the other, then yes there would be a signal strength difference but that isn't caused by the iPhone. Your best bet would be to go to each companies respective websites and checking out the service coverage maps for each and see which one has best coverage for the areas you need if all you are worried about is signal strength.
 
Like the others said it's all up to the coverage. However, with this said I honestly believe T-mobile is stronger. I notice in certain areas the AT&T service weakens significantly when you step inside certain buildings while T-Mobile does not falter at all. It pretty much rendered my iPhone useless whenever I was inside a building in Destin, FL while my friend's T-Mobile worked just fine.
 
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Small White Car said:
I honestly can't imagine using an iPhone without 3G.

Did it for a long time and it sucked. So happy my BFE community finally got 3G. For that reason alone, I'd go AT&T.
 
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