I think Szczesny is a fine goalkeeper,he's young but he got the temperament and arrogance. He seems like more of a winner than Fabianski and definately Almunia, and he's technically a very good keeper.
What De Gea has done so far in England isn't great, but so far is at the moment three matches.
What he has shown though, as evident against Spurs, is he's got a great technique of catching the ball without giving rebounds, he's fantastic with his feet, possibly even better than VdS in that aspect (

) and his distribution is very good. He also seems to have the balls to get over mistakes, if the Spurs-game is anything to go by.
He needs to work on getting down quickly, interestingly at times if Van der Sar did let in soft goals those would be the kind of goals he'd let in(although very rare, and he was good in general at getting down to the ground). De Gea is a very good shot stopper reaction-wise, as we know.
His biggest area of improvement is in the aerial work, and this is true in Szczesny's case as well, although the PIG could do with working on his distribution as well.
For me I think De Gea's got the bigger potential, but Wojciech looks a very good keeper at this moment in time and will only get better. Plus De Gea's the most expensive keeper in the british game so he bloody well should be talented.