Errr... there's LINQ for NHibernate.
Perhaps what you mean is which to use:
I prefer NHibernate.
LINQ to SQL is fairly lightweight, but it's a little bit more tightly coupled to your data structure, as opposed to NHibernate which is pretty flexible in terms of the types of object definitions that can be mapped to your table structures.
Of course that's not to say that LINQ to SQL has no uses: this very website uses it. I believe that it's quite useful to get up and running in small applications where the database schema is not as massive.
