I wouldn't say Ballard knows Langford any better than he does Odrick. And being familiar with the Colts system doesn't appear to matter to Ballard (see Jones, DJax, Walden, Robinson, etc.).
Odrick has played for two teams...the same number of teams that Langford played for before coming to the Colts. So he hasn't exactly "floated around" the past few years.
Langford did have a very good season when he first got here, but that was somewhat of an outlier. And that was with a different DC and before his injury. Yes, Odrick is also coming off an injury, but comparing their careers, Odrick has produced more consistently.
So if I had my choice, I am choosing the younger, more consistent (and better) Odrick...if the money is the same. I just think it would be a very shrewd move by Ballard if he could switch out Langford for Odrick without spending any additional money.