It depends on the read/write ratio of your database. If you are mainly a read-only database (80% reads or more) then I would suggest your mdf file be moved over there if on the other had you have a more typical 50/50 reads and writes I would suggest you move your ldf file over there. The reason for this is due to write-ahead logging inside SQL Server. For every data modification it's transaction must first be written to the transaction log (read: write to disk), but once your data pages have been read into memory then you are using the buffer pool to read from and write to (dirty page).