I think Song's good enough and will improve as well.Song is a ok defensive midfielder has imporved alot this season. But i'm not sure if he is good enough (especially againts big teams) to deployed as the lone DM in such games, like other premiership DMs like Essein, Mascherano, Hargreaves, Fletcher, De Jong, Barry. Melo would be a more commanding presence i reckon.
Plus i personally in the few times i have never seen Melo, most recent vs Inter Milan a few days go. I have never seen him look uncomfortable as a DM. Even so other options around Europe for that role could be Toulalan, M Sissoko, M Diarra or Keita -but of course not sure if any one of them would want to play for the gunners though.
Generally in the big games for Arsenal, the midfield hasn't been the main issue, it's been silly mistakes in defence or by the goalkeeper.
Those would be the pressing concerns for me if I was Wenger.