Not sure what you mean. Give or take, you get 5 years of play from the rookie for the same cost as 2 years of Sherman. His contract expires after 2018. Is he going to re-sign with us? Not necessarily. And if he did, would it be for $3-4m per year? No way. Re-signing the draftee is a concern that's too far in the future to worry about right now.
Sherman is good, but how many CBs maintain elite performance into their mid 30s? That's another concern I'd have.
I should clarify that I am not against acquiring Sherman, I just don't think the #10 pick is fair value for us.