Linq to SQL is an ORM, an Object Relationship Mapper. Entity Framework is also an ORM, and while it might seem that Entity Framework is a natural progression of L2S, they were actually developed in parallell and are quite different under the covers. For example L2S will not work with any other database other than MSSQL, where as the entity framework will work with most databases.
The entity framework allows you to use your own data provider, so the provider is going to rely heavily on which database you want to use and whether or not there is a provider available for it. If you have not yet chosen a database, you will need to weigh up the pros and cons first of each first (and then probably go with MSSQL because it's just easier if you're developing with .NET).
Given your preference form MSSQL then your database design will have a large impact on whether or not you wish to use an ORM. Linq to SQL L2S is still well used in the development community but I'm unsure as to it's future. Linq to Entities (or the entity framework) is relatively new, but as of 4.0 I believe it is much more business ready and well supported. You can also consider NHibernate if you like the open source (and hard work) option. It is harder work to get set up, but often worth the effort for complex domains.