I was bored so I made a scatter graph of Aguero and G. Rossi goals and appearances in their last 4 seasons. I included goals from all competitions and I got the data from wikipedia. Rossi is a year older but has one less year of experience in La liga in this seasons so I think it's a fair comparison.
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I think it's clear from the data that Aguero is the better player but also much more inconsistent. It's interesting that number of appearances and number of goals is negatively correlated for Aguero. He stagnated a bit in seasons 2008/09 and 2009/10 after a good season in 2007/08 but made a comeback this season. Interestingly he never scored as much as Rossi has this year. Rossi is the more consistent player.