I did some testing on this today, since our AT&T rep didn’t believe me. Verizon clearly won. This was testing from Las Vegas to the Switch pop in Las Vegas. Notice the extremely high AT&T ping times. Testing was performed minutes apart, browser was closed and cache emptied between tests. Testing was performed on a Dell Studio XPS 16, 4GB, i7. Nothing else was running in the background.
Verizon - USB – 760 – Modem – Firefox
AT&T – USBConnect Mercury – Firefox