Thiago. Because Hummels won't considerably improve the defense. He'll help with aerial balls and that will be the only improvement over Mascherano.
When Dortmund played at home against Madrid they took the initiative. Which means they left more space behind the defense. That's how Barcelona plays week in week out (even more offensive). The result? Hummels got exposed and cost Dortmund a goal. Just like he cost Germany when he defended Cassano like a schoolboy. In the second leg Dortmund sat deep, played on the counter and defended in numbers. That's when he put in a MOTM performance. And that's how Barcelona never plays like.
At Barca we would see the first leg Hummels more often than the second leg Hummels. Thiago Silva is a completely different animal. Age is no concern either. The most successful era in the club's history went from 2008 - 2011/12. The best defender in this era was Puyol. Now go and look up how old Puyol was in this time frame when he played the best football of his entire career.
If he's getable, throw out € 40 million for Silva without hesitation. Who cares if it's maybe € 10 million too much at the end? lol This is still Barca, not Arsenal. We have the weakest defense of any top team and some people want flavour of the month players like Inigo Martinez. Fss