Mignolet is and forever will be a more dependable goalkeeper than De Gea. I don't understand why people don't take into consideration that Simon has to save probably five times as many shots as David does, and let's be honest, doesn't concede that many more. Then there's the reliability side of things, De Gea makes probably a few too many mistakes in a season. Mignolet isn't perfect though, but he's better in this regard.
Both are great goalies, but Simon's a special player that's heavily underrated.