Obviously I'm not that objective, but I think he'd be crazy to leave. Staying with United, he'd have the chance to be involved in something truly special. What's more, he'd be revered by the fans, particularly when you consider his heroic performances during our transitional phase. Going to Real Madrid, he'd be joining a team already at the top, replacing a living legend in Casillas, and playing for the rivals of the team he came up with. It's very hard to imagine him ever being worshipped in the same way by Real fans, yet very easy to imagine him being blamed for any slips the team experiences. Even for Ronaldo, there have been turbulent moments, such is the demanding (or some might say, fickle,) nature of Real fans.
In the Ronaldo situation, it's easy to understand why he left: he was approaching the peak of his powers, and moving to a league in which scorers flourish made sense. As well, Real were in fairly bad shape (by their standards,) when he joined. None of that applies to De Gea.