Better signing for City has to be Isco. Like everyone has pointed out, City have a lot of CMs. Whereas they lack a player like Isco. He's not rapid but he has that ability to take on defenders and almost always succeeds. He also scores goals too - something City lack from midfield. I reckon if he has a good season, he will be able to score around 15 goals in all competitions.
Thiago is class too. I think he just edges Isco but that's because Thiago has this wonderful ability to seamlessly change from a Xavi-type player to a Iniesta type player in matches. It's very rare to find players like him, let alone young players. If Thiago was signed too (I doubt it), I reckon two centre mids would have to go and maybe Nasri.
As much as I would like him to play for City, I think he will go to where he has the ,most playing time, Utd, unfortunately. Maybe Txiki can persuade him, but it is looking like he is off to United for me. A shame really, he'd play much better in a system like City's. Especially under Pellegrini.
We'll see anyway. All reports/rumours point to Utd for Thiago but it was like that for Sneijder.
