If you mean that one side is a wall and the other side opens into your kitchen you should absorb on the side wall not diffuse. The open space into the kitchen acts like an absorber since sound get in there and gets "lost" and doesn't return to the listener. You would want to mimic this on the side wall which would be absorption.
If you mean a regular room with 3 walls and the kitchen behind you... I have heard of people doing both absorbing or diffusing or a combo panel. I think in a room your size it may be best to absorb to control reflections.
If you mean a regular room with 3 walls and the kitchen behind you... I have heard of people doing both absorbing or diffusing or a combo panel. I think in a room your size it may be best to absorb to control reflections.