actually you will be suprised at the humber of tackles barca make, its why they keep the ball so well. once a move breaks down its like watching a pack of dogs, they are so quick on the oppostion.
yaya is more complete, but in regards to the dm role at barca (which is not an expansive role at all) mascherano trumps him, hence an upgrade
but then again i will say iv never liked mascherano so i might be abit biased in that sence lol. They are both excellent players and city should be happy they got him.