LINQ in it self is a object query language (my very crude interpretation). there are different flavors of LINQ: LINQ to XML, LINQ to Sql, LINQ to ...
LINQ to SQL looks "pretty", and for simple read-only queries it's not a bad choice. However once you approach complex graphs and CUD operations LINQ to SQL falls apart.
SD is creating LINQ to LLBL (for lack of a better term). These objects are wrappers for LLBL objects. So you have a choice of using LINQ with LLBL, or straight LLBL. There is a thread around here somewhere I that explores LINQ more.