If they're both available then I think you almost have to take a chance on BOTH RB Lattimore and CB Tyrann Mathieu in the 7th rd.
Just like Vontaze Burfict last year. These are both 1st rd talents if not for off field issues or injury. You're not going to find that kind of value in the 7th rd often. If they don't pan out...wtf cares, they're 7th rounders...most 7th rd picks don't make it in the league anyways.
2015 Adopt-A-Chief: Derrick Johnson #56