Osvaldo might be a slightly better player but he'll still need time to integrate with the squad while quag is used to the team already. He might have more motivation to prove himself though especially that its a world-cup year. So overall, i dont think there will be any real difference in terms of football. I dislike his character though while I am indifferent about quags.
On the business side. Osvaldo is probably worth more especially if he rediscovers his old form. He is younger than Quags and has more marketability. I wouldnt pay 18 mill for Osvaldo no matter what unless he becomes the new diego milito.