Thiago Silva is one of the best defenders in the world, in my opinion he is the best. He is not only a great defender but just as important for Barca is that he is a leader which is something the team lacks without Puyol at the back. He would be a huge upgrade on a 35 year old Puyol and Masch, Pique would benefit from it as well just as David Luiz does playing alongside him for Brazil.
His age is not an issue and I don't understand why people keep mentioning it as he is only 28, which is about 6 months younger than Iniesta. If Barca put Iniesta up for sale this summer for 45million all the top teams in Europe would be fighting over him, his age wouldn't come into it. Iniesta is the best in his position and Thiago Silva is the best in his, both are good for another 4 or 5 years.
Marquinhos is a really good prospect but he may never reach the heights of Silva, I think it is the wrong decision to go for the younger player because he might be a better long term prospect. Barca need a top class CB now, someone who can slip right in alongside Pique and lead the defence. Marquinhos has had 1 good season and may fade like a lot of young players do, but if he has another great season at Roma, Barca can sign him next summer. Who knows, maybe next year there will be another young hotshot we should sign, this time last year I had never heard of Varane or Marquinhos.
Thiago Silva will not solve all defensive problems, but at least he is a natural CB in his prime which is a darn sight better than what we have at the moment.