Fluent NHIbernate sits on top of NHibernate, so its not really a choice between the two. If youre going to use NHibernate, CHOOSE to use Fluent NH on top of it to save yourself vast amounts of effort.
Fluent NHibernate is awesome, I wouldn’t use NHibernate without it. You can fluently map all your entities (giving you compile time checking, and automated testing support) instead of having to maintain cumbersome xml files and remember their syntax/DTD.
It can also automatically map your entities based on default and/or your own custom conventions.
Just use it!