Tevez>Aguero end of story. Tevez is simply the better player and is one of the few best fowards in the whole world. Aguero is not. Tevez is a big game player. Aguero is not half the big game player Tevez is. Tevez>Aguero. If you believe otherwise you're either: an idiot, illiterate, ignorant, egotisitcal or stubborn.
Tevez would also make a much bigger difference to this team than a CB or LB. If you've watched the game this season; despite the wins, you would see that this team is lacking a real clinical finisher upfront with very good first touches and link-up skills. Tevez is exactly that plus he's brings his immense workrate. Tevez would be the player that this team would play and that is not a bad thing at all for two reasons, #1: He is World Class and is extremely dangerous in attack #2 Him having a large role at the club would feed his ego and make him happy, therefore making him play better.