LINQ to SQL only supports 1 to 1 mapping of database tables, views, sprocs and functions available in Microsoft SQL Server. It's a great API to use for quick data access construction to relatively well designed SQL Server databases. LINQ2SQL was first released with C# 3.0 and .Net Framework 3.5.
LINQ to Entities (ADO.Net Entity Framework) is an ORM (Object Relational Mapper) API which allows for a broad definition of object domain models and their relationships to many different ADO.Net data providers. As such, you can mix and match a number of different database vendors, application servers or protocols to design an aggregated mash-up of objects which are constructed from a variety of tables, sources, services, etc. ADO.Net Framework was released with the .Net Framework 3.5 SP1.
This is a good introductory article on MSDN:
Introducing LINQ to Relational Data
So what questions should I by asking about the project and myself to help decide on the best mocking framework to use in a given case?
The questions you should be asking about the project is: Has the project been developed with the SOLID principles, or not? Is this a project that has loose coupling and high cohesion? Have good OO principles been utilized in building the project? Is a Dependency Injection container being utilized? Has the system been coded in a Design by Contract method (utilizing Interfaces thoroughly)?
If you answer yes to these questions, then you can utilize a mocking framework like RhinoMocks, which is what some would call an "opinionated" framework. RhinoMocks, and some other mocking frameworks, have very strong opinions about how a system should be designed in order for objects to be mocked. A framework like RhinoMocks expects your project to be designed a certain way. Mocking is certainly a lot easier with RhinoMocks when you've built your code the right way (no sealed classes, no statics, heavy use of interfaces, virtual on class methods, etc.)
If you answer no to those questions, or if you're working on a legacy system with a lot of highly coupled classes, then your only choice is going to be TypeMock, which can mock just about anything.
It would also be useful to know why you choose the mocking framework you are currently using and if you are still happy with that choose.
I chose RhinoMocks because at the time (3+ years ago) it was clearly the most mature mocking framework with the most features. I've stayed with it because it has evolved in away that makes my life much easier (the advent of the AutoMocking container being a gigantic step toward efficiency).
What I like about RhinoMocks, other than the feature set and ease of use, is that it guides me toward a better design in my code. I am not a perfect programmer, and I am going to make mistakes in design. But tools like RhinoMocks and NHibernate help guide me toward a better design, because when I do make mistakes and create poor design, these tools become painful to work with. NHibernate, for instance, is painful to work with if you have a bad database design. RhinoMocks is very painful to work with if you have a poor class design, aren't using interfaces, aren't using IoC... etc.
I like RhinoMocks because it ultimately helps me be a better developer, and not just because I'm testing my code, but because I'm shaping my code - designing it - in a better manner.
Best Answer
In a question of mine that touched the subject of DSL's, it was suggested to me to use a combination of Irony and DLR. I don't know stable they are however. Irony is an Alpha, and DLR, although stable, is still in v0.9. I'm aiming to start using Irony myself in a week or two, so then I will be able to provide some feedback. On the bright side however - they are both opensource, so you can fix bugs yourself.