Mid parries cover punches and kicks so you don't have to guess. The trade off is less damage, more recovery, and you cant use parries while stunned.
Parries have the ability to land more often. 3H parries all mid punches and mid kicks with +16 frame advantage. 9H parries all high punches and high kicks with +16 advantage. However, the most important attribute in parries... are setups.