What does "better" mean? You probably mean "better" in the "in any given game, which guy would perform better." Put this way, I think it's probably right that Tevez is overall a better player. And when he plays, he really wants it; Tevez has a will to win, real grit (although to be fair, Aguero is not much different this way).
But over the long term, over several seasons and long campaigns, I would rather have Aguero. You don't have the constant employment problems with him, frankly, that you have with Tevez, who is always homesick, wants to leave, go to another team, maybe to Spain, maybe back to Argentina, wants to play, doesn't want to play, etc. I don't mean to dismiss this as a personal matter for Tevez (for some reason, the Premier League is a tough cultural adjustment, let alone football adjustment, for many South American players, and it is terrible that his children don't live with him in England), but you're given a stark choice: (1) an incredible player who has a good chance of freaking out for say a whole season and not showing up much or (2) a somewhat less incredible player who can still, say, score in the last minute of the last game and win you the Premier League title, but is significantly more professional and stable in terms of his day to day life. I'd pick the latter as a coach/team owner any day.