[inquisitor]

For their 3G network T-Mobile use a frequency band called AWS on 1700 and 2100 MHz, which very few handsets support. As AWS is only being used in North America such handsets are even rarer outside North America. So before you consider T-Mobile for data make sure your device supports "UMTS 1700/2100" also know as "AWS". Afaik there's neither an AWS-compatible version of the HTC Desire nor of the HTC One XL, so T-Mobile seems to be ruled out for you. A phone that support only UMTS 2100 (which is a common frequency in Australia, Europe and Asia) will not work on an AWS-network as it lacks the 1700 MHz-capability required for the uplink.
Your two devices could only access T-Mobile 2G network which uses GSM 1900 but would provide low data rates. So if you need fast Internet access AT&T is your only option.