Of course Aguero is fantastic.... Cavani should not be compared with Aguero. They are different players.
We have Tevez and Aguero who both offer very similar things, and are extremely similar in build. Dzeko (in a perfect world) would be the idea foil for both of them... offering an aerial threat and a bit of muscle.... essentially a more skillful version of Andy Carroll.
Sadly, it seems that either Dzeko isn't quite as good as we'd hoped, or our style of play doesn't help him (or a combination of both).
What I've seen of Cavani, is more skillful AND physical player (than Dzeko) who appears to have more versatility to his game too.
I don't want to do Dzeko down, he's been a model pro and a very likeable player for City - but Cavani does appear to be a serious step up.
For the last two seasons, we've looked very like Arsenal of old... trying to pass the ball into the net far too often (a natural consequence of having a lot of possession, and teams having 9-10 men behind the ball). We've lost a lot of our counter attacking play, and to my mind, been short of some brute force in the box. A player like Cavani would (imo) resolve that issue.
He's very like Torres at his peak, and physical like Shearer was. Having him would probably open up more space for other strikers too.