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Polanco is a much better defensive 2B than Lopez and a better OBP. He was actually a better player in '09.
I'm not giving Lopez away. Hes a league average player making only a couple million, but for Danks? I don't get why its debatable. Its MUCH more difficult to find a potential 5 WAR SP than a 2.5 WAR 2B. If you want a slightly above-average 2B, theres tons of them this offseason.