inner join Vs scalar Function

Which of the following query is better... This is just an example, there are numerous situations, where I want the user name to be displayed instead of UserID

Select  EmailDate, B.EmployeeName as [UserName], EmailSubject
    from Trn_Misc_Email as A
         inner join 
         Mst_Users as B on A.CreatedUserID = B.EmployeeLoginName

or

Select  EmailDate, GetUserName(CreatedUserID) as [UserName], EmailSubject
    from Trn_Misc_Email

If there is no performance benefit in using the First, I would prefer using the second... I would be having around 2000 records in User Table and 100k records in email table...

Thanks