Yeah, they pretty much are a wash.
Webb was awful in 2011, but did up his game some in 2012... Not to a point where he was a good OT, but better.
Since 2011 Webb has allowed a QB sack, hit or hurry on 4.2% of the snaps he's participated in, based on 2252 snaps.
Since 2011 Parnell has allowed a QB sack, hit or hurry on 4.6% of the snaps he's participated in, based on 342 snaps.
There really isn't a big difference between the two... I'm thinking that the Cowboys believe Parnell has a higher ceiling that he can still reach where Webb has already reached his.