There are mutliple dimensions to consider: On the negative side, SQL Azure is relatively expensive for gigabyte of storage, does not scale super well and is limited to 150gigs/database, however, and this is very important, there are no transaction fees against SQL azure and your developers already know how to code against it.
ATS is a different animal all together. Capeable of megascalability, it is dirt cheap to store, but gets expensive to frequently access. It also requires significant amount of CPU power from your nodes to manipulate. It baiscally forces your compute nodes to become mini-db servers as the delegation of all relational activity is turned over to them.
So, in my opinion, frequently accessed data that does not need huge scalability and is not super large in size should be destined for SQL Azure, otherwise Azure Table Services.