Performance may vary. Just like the base voice network, the effectiveness of your 3G signal will depend on how saturated each repsective carrier's 3G signal is where you want to use it.
They also run on different base technologies (CDMA for Verizon, GSM for ATT), and different frequencies. So, if you are thinking of unlocking a device from one carrier to use on the other carrier's network, it won't fly.
Hope this helps.
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In addition to what ndub33 stated, GSM 3G allows for simultaneous voice and data where as CDMA 3G does not.
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Is 3G the same for CDMA & GSM? 
