I just finished the cabinets in my shop and built them with pocket screws in 3/4" plywood. The backs were 1/2" ply in a dado with glue and staples. I have built them with dado, screws from the outside and biscuits at various times. I would say that any or all would work and depends on what you like and what is easiest for you. With pocket screws, you have to make certain to hold the pieces tight and in place when putting in the screws. With biscuits, you have to make certain to get the pocket properly located. The cheapest is probably assembly screws from the outside and works just fine.