<p>What are in your opinion big differences between areas of usage for "Astoria" (ADO.NET data services) and SQL Server Data Services?</p>

They are similiar but very different technologies. Astoria or what is now called at
Microsoft ADO.NET Data Services
is a programming library that will allow data to be passed through RESTful web services.  You develop these web services to be run against data you have access to.  ADO.NET Data Services is now included in the .NET 3.5 SP1 updates.
SQL Server Data Services
is a new service provided by Microsoft.  The following is a decription:
"SQL Server Data Services (SSDS) are highly scalable, on-demand data
  storage and query processing utility
  services. Built on robust SQL Server
  database and Windows Server
  technologies, these services provide
  high availability, security and
  support standards-based web interfaces
  for easy programming and quick
  provisioning."
SQL Server Data Services is very similair to Amazon S3 service.