I have never been one for a list of letters after a name!
It depends on what shooting skill and level you are aiming for? If it is skeet/clay then an experienced coach in this field with the right tempremant and manner would be best.
If it is rough or driven game shooting then the techniques are different.
My Grandfather always said 'you can't teach experience'
I suppose above all you have to actually like your coach as a person first,if the personalities dont get on you won't learn in any field.