There are various reasons that some might consider EF better than NHibernate, the fact that EF is linked to releases of the .NET framework is not really one of them. This works the other way too - if, when released with the framework, people have major objections to EF, users have to wait at least 18 months for the new version. NHibernate, while not always taking advantage of latest language features, can release at its own pace (dictated, incidentally, by how many people are able to contribute). Broadly, they do perform the same function in a system, how they go about is very differently though.
I will say without a minute's hesitation that NHibernate is the better one to jump into. This strong opinion is formed on my own, though, and is based on criteria that you might not care about. Entity Framework has some interesting features, and has potential, but as it stands I cannot work with it, nor do I want to. V2 looks better, but NHibernate is the best ORM, hands down.
In a nutshell, try them both, and form your own opinions. Read about persistence ignorance (PI), and decide whether it's something important to you. Consider whether you're a drag-and-drop guy, and how important things like designers are to you. Consider whether having access to the source (real access, live and unfettered) is important to you. Understand the various layers in EF, and what they're doing, and why.
Once you start understanding ORM tools, and decisions the designers need make (and EF and NH give you two good points for comparison) you'll be set.
The high road might seem longer at first, but you'll enjoy the downhill at the end...
http://blog.benhartonline.com/