I am extremely sceptical about this 'bid'. But, if it is genuine, I would take it without hesitation and go all out for Aguero.
I have never believed that City would be inundated with serious interest in Tevez. As a result of a very prolific spell from December 2009 to December 2010, Tevez has been overrated - certainly in terms of his world standing - both by many City supporters and by many English football journalists (who have delighted in portraying City as a one-man-band). I do not believe that the owners and managers of 'top' foreign teams see Tevez in the same light. European clubs are certain now to be wary of a player whose clear modus operandi is to sign a 5 year contract and then press for a move after 2 years (whilst insisting that he has always given 110% for the club and that he will always love the fans) and who has recently reiterated that he may well retire from football in two or three years. Moreover, intelligent managers will surely by now realise that Tevez has recurrent problems with authority figures - in particular, he reacts very badly to being substituted - and that he is inordinately selfish, so much so that he sees other strikers as rivals.
Comparisons with the inflated transfer fees for Torres, Carroll, etc have very little relevance. If City can get a transfer fee for Tevez that is close to the buy-out price for Aguero, that must be a good deal.