Windows Azure Table is focused on structured storage at massive scale and at low cost without the relational database features (e.g., no joins). This will allow you to have structured storage with entities and properties, where you can query over any combination of properties using LINQ, and at some point in the future perform atomic transactions (entity group transactions) across multiple entities within the same partition. But Windows Azure Table is not a relational database in that it does not provide joins, or automatic maintenance of foreign keys across multiple tables, etc.
SQL Data Services is focused on providing a premium service with the relational database functionality, and it will over time expose the traditional relational database capabilities like joins.
If you really need the relational database features (joins, auto-maintenance of foreign keys, etc) for your application then you should look at using SQL Data Services, if you do not, then you should look at using Windows Azure Tables.