I will take Chivu over Mexes too. And I would swap Trez with Chivu.
Right now, I think with likes of Bianchi, Huntelaar, Saviola, Pizarro on the wings, who can come in pretty cheap, they are players who gurantees goal even if they may not be prolificc as Trezeguet. But players of Chivu is really in short supply ATM. Watching likes of Maldini, Nesta, Canna, Matrix and Costacourt, I think the shelf-life of defender these days are far longer than before, thus if we buy Chivu right now, we could have a defender next decade or so. In contrast, I think Davids has only couple of good years left in the tank, and surely now is the last time we can get reasonable financial return on his sale. All said and done, I do think we should keep Trez, and Roma won't give away Chivu, when they are team are couple of signigns away from being a big boy at an European stage. But if contract extension isnt going too smoothly and Roma are interested in such offer, then we should defintely do this deal.