It depends on the opponent we face..Against United,Chivu would be the most appropriate player to mark Ronaldo since he's by far a better defender than Maxwell...Most of us here,have already mentioned that Maxwell is not that good defensively,and that was proved against Siena as well.Ronaldo can easily bypass Maxwell and mistakes against him are forbidden in such game...And as a pure defender,Chivu has faced many great strikers all these years and can take care of Ronaldo in my opinion..More experienced with better vision,can be of great help against top sides when we need to defend..
Also,Chivu can play as a defensive midfielder,and create our offensive play from behind,providing more cover to our offensive midfielders like Stankovic and Muntari..
Maxwell should be used against theoritically "easier" teams when we need to play very offensively with crosses from both flanks and quick movements up and down the left flank..His crossing is better than Chivu's as well as his speed..
Against United,a defence line with Maicon-Cordoba-Samuel-Chivu can handle United's guns better IMO..