Dos Santos is not a natural winger although he can play there. Navas is a natural winger but only on the right.
I'd prefer Dos Santos any day of the week tbh. He's not only more versatile, he's much better too. And he's much more adaptible. You're playing in Spain so you shouldnt have a problem with Navas. But he generally grows restless if he's asked to move countries (Arsenal fans, remember Reyes!?!

). So I'd rather a little more money and get Dos Santos. But will he actually come to you though? With his love affair with Barca and Rijkaard, it seems he's never willing to leave the damn club!
If you have the money, I'd recommend Cristiano Ronaldo for the wings. He's just a magician mate. 10 times better than bloody Navas.