Coquelin's passing, for me, is twice as good as Songs, because he passes it early, accurately and with real zip on it into better creative players than himself. Song took an age to release the ball and often tried to be too clever when he rarely had the vision or technique to justify it.
Also feel Song's tackling was lazy, lazy, lazy. So often he'd stick out a leg from the wrong side of his man because he couldn't be bothered to get the right side and tackle properly. Coquelin is much more focused and aggressive, even if he does mistime a few from time to time. I also think Coquelin reads the game twenty times better than Song ever did and their positioning is incompabale as a result.
In my eyes, Song as a holding midfielder was bearable, even with his annoying faults, Song as a two way midfielder, I couldn't stomach and was glad when it ended. Coquelin is far superior in the holding role, for my money.