SQL Server 2012 :: Explicit Casting Varchar And Nvarchar Columns?

I know that if I have an nvarchar column I can use an equality like = N'supersqlstring' so it doesn't implicit cast as a varchar, like if I were to do ='supersqlstring'. And then I'll be a big SQL hero and all my stored procedures will run before a millisecond can whisper.

But if I'm comparing an nvarchar column to a varchar column, is it better to cast the varchar 'up' to an nvarchar or cast the nvarchar 'down' to a varchar?

For instance:

cast(a.varchar as nvarchar(100)) = an.nvarchar

or

cast(an.nvarchar as varchar(100)) = a.varchar

Leaving aside non-matching, like (at least I don't think) that SQL considers the varchar n to be equal to the nvarchar ń, what's the best way to handle this?

Pretend for a moment that each column contains a mixed letter and number ID with no accented or wiggly-squiggly Unicode characters; it's just designs clashing.

Is there a performance hitch doing it one way or another? Should I use COLLATE? Should one of the columns be altered?