Good diffusers will be much more effective than bookshelves by producing a 3-dimensional airy sound in the midrange and treble. That’s the main difference when introducing diffusion in the room.
Sound-wise diffusers will win. Practicality and aesthetically bookshelves have the advantage. Although they might not bring a large impact as proper diffusers, they still bring some and it's better than none. On the scale of 10, if diffusers score 9, bookshelves score 4 or 3. Better than none.