Buy the global GSM it is optimized for getting 3g data when in roaming while the US GSM will give you 3g when you are inside USA. You might not get 3g while on roaming on US GSM.
ATT sim works perfectly on Global GSM and you will also get 3g outside us.
These are the bands which are supported by either versions.
Global GSM Model:
GSM/GPRS/EDGE (850, 900, 1800, 1900 MHz)
UMTS/HSPA+ up to 21 Mbps (850, 900, 1900, 2100 MHz)
US GSM Model:
GSM/GPRS/EDGE (850, 900, 1800, 1900 MHz)
UMTS/HSPA+ up to 21 Mbps (850, 1700 (AWS), 1900 MHz)
The only problem will occurs when using T-mobile on Global GSM.
source- Motorola forums.