Pekovic is a great center and he'd help us out a lot if we got him. I would be ecstatic if the Celtics traded for him. He's a great finisher, a good defender, and will give you the "Perk" mean interior defense factor. (Perk-Pek...get it?
That being said, Cousins is already the better player and has the potential to be so much better. He's the better offensive talent and the better defensive player. He'd help the Celtics out more than Pek, I would think. Then take into account his potential.
And from what I recall, Paul Westphal, the Kings coach who had a winning percentage of .298, was fired right after he claimed that Cousins wanted a trade.