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.