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.
__________________ 1st Step in Troubleshooting: Do you have a BlackBerry Data Plan? 2nd Step in Troubleshooting: Pull the Battery.
In addition to what ndub33 stated, GSM 3G allows for simultaneous voice and data where as CDMA 3G does not.
__________________ The difference between stupidity and genius is that genius has its limits. When you take things for granted, the things you are granted, get taken. Even a mosquito doesn't get a pat on the back until it starts to work. Too many people miss the silver lining because they're expecting gold. [BES 5.0.3 / GroupWise 2012 HP2]