Flashbang:
The flashbang effect's last alot longer than the stun, But the flashbang has to hit really REALLY close to the enemy to show the crossairs up on the screen.
Stun:
With the stun its that little more effective than the flashband simply because it slow's down the enemy and you dont need to get it as close to the enemy to know its hit them.
So my vote is for the stun