It was a communication failure the Carling Cup, which I usually think is a goal keeper responsibility. Szczesny is alright but he hasn't set the world alight really, he's solid but not really game winning in the same way that top keepers can be. He may end up being a great keeper, but does anyone think he is the finished article? And if not, why would a club like Arsenal use him yet? They can buy better and loan out Szczesny until he is more experienced.
As for De Gea, I've seen nothing of him yet so I look forward to seeing how he does next season. Fergie doesn't always get the keeper signings correct though. What I'd usually say is that experience is a very valuable asset for a keeper though and obviously he doesn't have a great deal of that yet.