Taking Pellegrini's achievements at city is pretty pointless, cos they are playing the league on a different level to the majority of teams. Winning the league with City doesn't automatically make you a great coach, actually his best achievement at City was getting them to the semi finals of the CL.
It's what he did with teams before and after them and Real Madrid, that is where the focus should be. I think I said this on another post here, that he had far more quality to work with at these teams too (Malaga and Villareal) than he he will likely be starting with at West Ham, so I can see it being a very tough job for him.
If it was my team, I would rather have been going after Silva than him. Silva may not be an 'illustrous' name in comparison to Pellegrini at this point, but he's a coach who's on an upward trajectory, whereas Pellegrini isn't in my mind.