What Active Directory field do I use to uniquely identify a user?

I have an Asp.net MVC project authenticating through AD. I would like to store audit information in tables for the current logged in user. What should I be storing in the database? I am currently using SamAccountName for my membership and role providers. Should I be using this? Should I use the more verbose and modern UserPrincipalName? What if we eventually end up using multiple domains?

ActiveDirectory Current Username in ASP.NET

I’m trying to get both ActiveDirectory and standard forms login working but one thing is stopping me. I can’t get the name of the current windows user. The closest I’ve got is var i = WindowsIdentity.GetCurrent();, but that gives me the name of the IIS app pool user. I have Anonymous Authentication, Forms Authentication and Windows Authentication enabled in IIS. I can load users from AD so I assume my web.config is setup correctly.