I think its an unfair comparison, Aguero has never played on the same level as Tevez has. Tevez has played with Manchester United and won a Champions League final, a Club World Cup and the English Premier League on two seperate occasions. He also made the transition from Manchester United to Manchester City and also managed to play at a high standard.
On the other hand Sergio Aguero has never played on the same club level as Tevez, admitedly he did win the Uefa Europa League with Athletico Madrid but thats a world apart from the pressure Tevez has had to deal with.
However if I had to choose between them I would have to go for Aguero, for the sole reason that Tevez often says how he doesnt love the game anymore and sways back and forth with statments claiming its because of his family or just because he has no passion left for the game.