I would agree with @this. __curious_geek that choosing the right ORM would depend on your requirements.Having worked on both Hibernate and Entity Framework, I feel the latter is more user friendly as it is a GUI based editor. On the feature richness front, NHibernate has an advantage of supporting a lot of database providers. Also customizing NHibernate turned out to be much easier than tweaking the Entity Framework.
Assuming the most tools satisfy your core requirement, I would prefer NHibernate as a vibrant and engaging user community is a big plus for any tool.
