I would keep Lidge. As some have said, Dice-K will probably regress. Lidge was a top closer with a real nice K rate.
But my main reason for keeping Lidge would be the crazy closer scene this coming year. Some would say that this will lead to an easier time getting saves fron the waiver wire , but there will be many managers seeking out these saves.
And another thing is that there's many good starting pitchers this year. I think it's a pretty deep position , with many up and coming possibilities.
So why trade a shallow position for a deep one ? I normally am hesitant about keeping closers, but in your case I would definately keep Lidge.
Dan the Bluesman