I'd have to say de Gea, just because of the age. From Valdes you can get some 3-5 seasons, while from De Gea 13-15.
Plus, De Gea is probably the best young goalkeeper in the game, only a regen might have better potential, so that's another reason right there.
And finally, I don't think the wages they'll ask are even comparable, with De Gea being by far a cheaper option.