Speed is obviously the number one thing you should look at when deciding where to stick your Azure services/resources. The faster the speed, the better the performance. Want to know which data center has the lowest latency from you? (tested against blob storage) Check this out link – http://azurespeedtest.azurewebsites.net/
However, please note that not all services are available in all Azure regions, this is actually one of the deciding factors of how we chose which Azure Data Center to be in.
For example, in Europe, features appear first in the West Europe (Netherlands) before North Europe (Ireland). So, if you want to be on the bleeding edge, this might be a deciding factor.
What to know what is available where? Check this out – http://azure.microsoft.com/en-us/regions#services
Hope this helps!!