I think this is a classic case of McKay/Mora wanted to bring in their "own" guy. Now Brooking is the longest tenured Falcon player and the last one to be here since the Super Bowl.
Hall is a quality vet that I think could still start on some teams in this league. Mitchell is a decent player, but isn't quite up to that level. Hall made 6.5 plays behind the line of scrimmage last year (3 sacks, 3.5 stuffs). Mitchell made just 1.5 (1 sack, 0.5 stuffs). That right there indicates how much more of a playmaker Hall is.
"Vincere scis, Hannibal, victoria uti nescis" -- Maharbal, 216 B.C.E.