Leaning towards ILB on de Gea.
It's a massive step up to make at that age. If he makes mistakes for Atletico they're not as noticeable as they're not challenging at the top where the margins are tighter and the differences between success and failure aren't as exaggerated.
I can see your reasoning, it will be a different kettle of fish playing for United. Many keepers find it different in the spotlight with little to do, but with higher importance when the time comes. I suspected United would probably go for Stekelenburg, as he has more experience and has already demonstrated so, on bigger stages. I did think maybe Fergie was erring on the side of potential, rather than demonstrated ability.
There is also an effect from the keeper that reverbarates throught the defence. Whether it be a calm reassurance in certain situations or understanding when they will rush out and adapted accordingly, something Howard struggled with, but we seem to have adapted to, where United didn't. Gomes for instance seems to confuse the defence and leaves them unsure what he will do next, even though he seems to be one of the best in many other areas.
I can understand caution over De Gea's ability to fit in and adapt, but for me, Szczesny has already shown a lack of understanding and communication with defenders and an eagerness to get involved when he doesn't need to. These are things he can improve, but I don't think they will come easily or soon. De Gea is familiar with his current defence, how good his English is and how he works with the United defenders will be important. But the caution is the unknown, he hasn't been in those situations yet, so it's hard to tell beyond what capabilities he has already shown in his current situation.
I never said De Gea was guaranteed greatness, but I think there is certainly more potential than Szczesny, considering Szczesny has already demonstrated critical frailties, with several at crucial times.
As ILB said though, De Gea's people have come out and said nothing is completed as yet.