Replicating - .BAK vs .MDF/.LDF

Hi,

I am tasked to replicate an SQL Server database to a different machine. From SQL Server 2005 (Win Server 2003) to SQL Server 2012 (Win Server 2012).

From the server I saw there is a *.bak file being generated daily as a backup, is *.bak an Online backup? But I am thinking to perform an offline copy of *.mdf and *.ldf files.

I would like to know what is the difference if I were to:

1) "Restore" the DB using the .bak file?

2) "Attach" the DB using .mdf and .ldf file?

Which is the "better/correct" way to replicate the DB, without data lost?

Thanks in advanced!