if it was me, I would certainly use the ply.
apart from cheapness, I don't know any advantage in chipboard.
Also, the bigger boards have fewer joints, which is an advantage. Put all cut edges under the skirting, and the square factory cuts will butt tightly together. Stagger the two rows. Use countersunk screws and mark pipe and cable runs on the upper face.
You may want to tighten the screws after a couple of weeks, though this is I think more common with chipboard.
I included a floorboard safe.
apart from cheapness, I don't know any advantage in chipboard.
Also, the bigger boards have fewer joints, which is an advantage. Put all cut edges under the skirting, and the square factory cuts will butt tightly together. Stagger the two rows. Use countersunk screws and mark pipe and cable runs on the upper face.
You may want to tighten the screws after a couple of weeks, though this is I think more common with chipboard.
I included a floorboard safe.