Linq is:
- a "proof-of-concept" done by the Visual C# team to show off the capabilities of Linq
- a straight 1:1 mapper - one table becomes one entity in your code
- for SQL Server only
- not very well suited to support stored procedures (you cannot e.g. create "complex types" to mirror values returned from your stored procedure)
- designer-driven, database-first only approach (and model cannot be easily updated if your database changes)
- basically a dead-end technology - there might be bug fixes here and there, but certainly no new features; it works - but don't expect any further development on this
--> so Linq-to-SQL works, and quite well in .NET 3.5 - but don't expect anything new here....
Entity Framework (at least in .NET v4 and up) is:
- a "proper" OR-mapper technology (and more) done by the ADO.NET/database teams at Microsoft
- a flexible mapper with a physical layer (database schema), a conceptual layer (your .NET objects), and a mapping layer between those two (three-layer approach)
- supports several databases (SQL Server, Oracle etc.) out of the box - fairly easy to write an Entity Framework compatible provider for other databases
- supports stored procedures very well (you can even pick a stored proc for one entity and one operation, e.g. for the DELETE)
- offer database-first, model-first and code-first development approaches
- if using model - that model can be updated from the database if your tables change over time
- the product that Microsoft is investing lots of their resources into - still being very actively developed (additional features, new approachs like code-first development etc.)
