I'd say the fact that Entity Framework is from Microsoft is both an advantage and a disadvantage. You're getting a framework right from the same source as .NET itself. The bad news is that Microsoft often obsoletes its own code base without regard for backwards compatibility.
NHibernate does not come from Microsoft, and there's no standard other than what the developers of Hibernate and NHibernate say belong in their code. The good news is that they've generally paid attention to backwards compatibility. There's a large user base, because Hibernate has been around for a while.
One of the features I like best about Spring is that they don't take decisions like this away from you. Spring has its own JDBC features, but it supports Hibernate, TopLink, JDO, iBatis, and JPA. You're also free to inject your own classes if you decide to go another way (e.g., NoSQL). Why should your choices be dictated by the framework choice? It's very nanny-ish: "We're Microsoft; we know what's best for you better than you do."
