ASC tube traps offer both. The tubes have a partial reflective surface identified by a silver dot. You can change from full absorption to a combination absorption/reflection by rotating the orientation of the trap. I use 2 11x36 and 1 9x72 on the speaker wall. I use the two nearest the speakers with the reflective area towards the listening position and the traps at the back of the room in the corners in a more absorptive orientation. The changes wrought by orientation are readily discernable. My answer then would be both absorption and diffusion. On the down side, they are very pricey.
