Diaby isn't the quickest passer and that's a totally valid criticism, but his discipline, athleticism, and control have been top notch. Sure he fluffed a good chance (something Ramsey never does, btw :). Diaby hasn't played with Arteta, Cazorla, or Jenkison in real games almost at all, so he needs just as much time as they do to settle into the new status quo. Passing will likely get a bit faster as understanding grows, though he'll never be the speediest passer in the squad by a long shot.
He's helped dictate our play almost as much as Art has these past 2 games, and he's doing exactly what a great number of us on this forum have been asking Song to do these past 2 years. If he stays fit and continues to play and get back into the game, he'll be far more important to this year's team than Ramsey is even capable of at this point in his career. I'd back him to score a few goals along the way at that. That being said, Wilshere will likely take his spot when he gets back up to speed. We just don't know when that will be.
Diaby>>>>>Ramsey for me. Far prefer the Frenchman to start against 'pool at this point. He's something our attacking players can build off of and learn to trust (until he breaks, that is). Ramsey is all over the place, even if he may one day be a far better player. He seems so much better suited to be a sub as he offers that extra dynamism that can confuse defenders later in the game when we're looking for a winner.
Just my 2 cents. I can totally see why people don't like watching Diaby play. He's one of those players that almost invariably produces a very subjective response.