It seems like most top players are using stun grenades over flash but I don't understand why. Stun throws a little quicker but flash seems so much more debilitating in most cases. Flash makes you blind, lasts longer, and has a larger area of effect.

Flipping it around, if I'm camping an area and get stunned, I'm often still able to get the kill. Mobility is crushed though so if someone is knifing or throws a grenade/semtex immediately after, that's a tough followup. Generally though, I think flash is much better. You just have to make sure that you don't blind yourself (i.e. bounce it or take cover until it explodes)