Tevez is only leaving City because he wants to be with his family, not for sporting reasons where the club has just won the FA Cup and is in the Champion's League. There is also nothing to indicate Tevez and Aguero are even close, so I doubt Aguero would be using Tevez as a way to asses his future.
As for Chelsea, Anelka is leaving and Drogba is likely to follow. They need another striker and Aguero perfectly fits the bill.
Personally, I think either Man City or Chelsea are leading the race for Aguero. Juve can't compete with them for cash, chance to win trophies or the prestige of playing in a big league.
Rossi is obviously plan B. But will Marrotta pay the 30 million needed to get him? Barcelona couldn't get Villarreal to lower their demands so I doubt he can. Even if he did Rossi isn't the big name promised.