Duplicate:
What OR\M would you recommend for a large ASP.NET application, Entity Framework or NHibernate. What do you consider to be the advantages and disadvantages specific to both technologies?
NHibernate is much more mature, has a large userbase, and supports 'real lazy loading', is persistence ignorant...
I would go for NHibernate.
I put together a fairly lengthy answer to a similar question: ADO.NET Entity Framework: Decision Making between ORM solutions.
However, since you asked :) I agree, I'd pick NHibernate for an "Enterprise" web solution today, however..
The next version of the Entity Framework should be worth taking a long hard look at. The first version wasn't ready for large scale deployment and the generated T-SQL leaves a lot to be desired, but I see a big future if the ADOEF team can put it together correctly. It could be really something, especially if it's able to consume the revamped SQL Data Services!
If anything, I'd brush up on eSql (Entity SQL) and get used to the syntax for using LINQ to write data queries. Something tells me this will be excellent experience to have handy for the next few years.