I got the same error.
Here is the conversion :
(ISNULL(Mobile)?"": (DT_WSTR,1024)Mobile) != (ISNULL([Dest_Mobile] )?"": (DT_WSTR,1024) [Dest_Mobile] )
Below article is nice for reference
http://sqlblog.com/blogs/andy_leonard/archive/2009/02/04/ssis-expression-language-and-the-derived-column-transformation.aspx