They both will produce the exact same result (create a string array) as in the duplicate post that sstan referred to.
But there's only one difference I can think of, which is: you can directly specify the number of elements that the array will consist of when using the first method..
Dim MyVar() As String 'An array of string (the number of elements aren't yet specified).
Dim MyVar As String() 'Same as above.
Dim MyVar(5) As String 'An array of string (consists of 6 elements).
Dim MyVar As String(5) 'Wrong (you'll get an error).
Hope that helps :)