(I tried to answer non-standard, if possible.)
The most important ORM selection criteria are your business requirements.
I would say that another important criterion is your belief in the future sustainability of ORM. Microsoft tends to change its data access technologies every two years or so, so I would say that the future of the Entity Framework is unclear. NHibernate can follow the same path as NUnit - when Microsoft released MSTest, NUnit languished for a while, but now NUnit has momentum again, since Microsoft is largely neglecting MSTest. You have to come to your own conclusion here, right now Microsoft seems to be very fond of the Entity Framework, but as far as I know, it still doesn't handle enums very well, and I haven't heard when this could change.
Another criterion is whether you prefer technology or open source technology from Microsoft or commercial third-party technologies.
At some point, the features of ORM will make a difference. You may need a prototype script with multiple ORMs to determine if the functions (and / or performance) of the ORM meet your requirements. NHibernate is the most functional ORM in the .NET space.
Do you need to support a specific database provider? Choose an ORM that supports the one you need. SQL Server is the most widely used database provider in the .NET world.
Do you have a preference between the first code or the first database project? Choose an ORM that supports the one you need.
Do you have a preference between an active recording and a repository against something else? Choose an ORM that supports the one you need.
I answered the question below that you selected .NET ORM in part, providing links to all cases when people asked which .NET ORM they should use in StackOverflow. There is great value to be gained from reading these other answers.
NHibernate, Entity Framework, Active Records, or linq2sql
There is not a single correct answer, not a single correct set of criteria for selection. Your business requirements are the most important component to consider.