Agüero is a genuine striker. Alexis is a winger / supporting striker. Agüero's finishing is far superior. He has a 1 goal in 2 games record over 230 games at Atletico (16 goals in 25 PL games so far) and he started playing there at age 18 when he obviously wasn't as prolific as nowadays (he's still only 23). I think Agüero is the best CF in the world today. I have to disagree that he wouldn't fit Barcelona. He's very similar to Villa (with much better dribbling ability). He could play on our wing as an inside forward like a boss. Henry did it, Eto'o did it, Villa does it. So can Agüero (in fact, he does for City with either Balotelli or Dzeko on the top).
Sanchez was known for his dribbling and trickery. He completed the most successful dribbles last season in Serie A. 3 per game. Pep always repeated that he likes Alexis' ability to get past defenders. Somehow he doesn't do it anymore for us though. His dribbles have dropped to 0.8 per game ... which isn't any better than Villa's dribbling stats who got heavily criticised for not taking on his defender last season. We somehow use Alexis centrally, as a CF, now. Not so much as a winger.
Not that I complain about da Sex. I wouldn't have complained about Kun either though. He handed in a transfer request and Atletico obviously didn't want to sell to Real Madrid. He wanted to play for us and with Messi. Negotiated properly, we could have gotten him for only a bit more than what we paid for Alexis.
Agüero-----Messi-----Villa
It would be like our treble winning Henry-Messi-Eto'o frontline. Just better.