My understanding on ATT is if you have the phone on and a call comes thru, you get charged for roaming whether you answer it or not. I may be incorrect. They will not charge you for having the device on, just when a call comes thru. As for cost, I'm not sure that there is that much difference overall, typically ATT is a few dollars more expensive but you do get better coverage in the states, so I could see why corporate may pick ATT over T-Mo for service in the US.
For T-Mo, I used the phone with no voice plan, just the 20cent/min plan and the number was not known to anyone so I did not incur any additional charges.
As to compare the 2 internationally, I would guess that they would share the same roaming partners. In China, I know that TMo uses China Mobile and would assume that since they are the largest and already have the infrastructure in place, ATT would also roam with them. Essentially, it would be the same service. If you travel to Europe, remember that T-Mo is huge over there and they have a huge native footprint.