Again, evaluating individual player production is very, very hard in soccer, but let's give it a whirl nonetheless. I took a look at Aguero's past five seasons in La Liga and compared them to the EPL stats of the man he is ostensibly going to replace: Carlos Tevez.
The difference in games started seems to be a function of Tevez's being rotated in a bigger squad at United (he had 11 substitute appearances) and playing only half of a season for West Ham in 2007, rather than any injury history. For the purpose of this analysis, we can assume that both are fairly injury-free.
Tevez's goals per game started ratio is slightly higher than Aguero's (.53 to .49), but he scores less efficiently than Aguero. Aguero's Finish Rate of 36% is higher than Tevez's 33%. So far though, there is not a demonstrable difference in quality.
We do have to adjust for league quality though. Over the past 5 years, Spain and England have been the top two teams in UEFA coefficients, not much difference there. Scoring is also similar between the two leagues, so goal totals are not inflated the way they are in, say, the Netherlands.
Team differences though are where it gets interesting. Atletico have long been a team accused of all attack and no defense, whereas City have been branded with the opposite label. Are Aguero's goals simply the function of the style of team he played for and will his production subsequently drop when he moves to a more defensive-oriented team?
Since I don't have a ton of data for La Liga, I used total goals (for and against) to proxy for the style of the team. Last year, Atletico had 115 total goals, fifth in La Liga, hence representative of a fairly attack-minded team. City had 93 total goals, second to last in the EPL, and thus representative of a fairly defensive team. It stands to reason that Aguero might lose some opportunities to score because of the change in style. However, he might well gain some opportunities being more of a focal point in the attack than he was at Atletico (Tevez almost certainly earned a greater share of the team's shots on goal by playing a lone striker). Moreover, his rate stats suggest that his totals are not wildly inflated by taking a ton of shots; he can convert the chances he gets. He also didn't take penalties for Atletico last year (except for one) and once that is taken into account he looks better still.
All in all, I find myself thinking that Aguero will be a quality player for City. There might be problems with this deal, but Aguero himself should not be one of them.