NHibernate may be more mature. That does not necessarily mean it is a “better” solution. Having used it at my job for some time, I would personally prefer to use almost anything than NHibernate (even straight SQL, if migration were remotely feasible). The number of error messages thrown by NHibernate that don’t mean anything (or that do mean something but should never occur) is absolutely staggering, as are some of its default behaviours (such as flushing the session once for each object returned in a Find).
Personally, when I have a choice, I use LINQ to SQL for all database work.