Sinfield has contributed far more to Leeds than Peacock. I'd say Sinfield.

I'd go for Sinfield too. That's not disrespecting Peacock, he's been immense for us, and I'm sure we wouldn't have won as much as we have without him.
But Sinfield is the man the team and, crucially, the mentality at Leeds was built around. Obviously with very important additions such as Peacock, McGuire, Burrow & JJB (who often gets overlooked) but Sinfield is/was the main man in my opinion.

In the run up to the cup final there was that story in the paper about the sad passing of his father and how he played against Wigan, whilst he was on his deathbed (winning MOM) and then heading back to spend the last hours with him.

In that story it encapsulates the mental strength and sheer bloody mindedness that has made him a definite great of the SL era.